NOW the euphoria that followed the announcement of Sino-British talks has died down, the question must be asked: what is there to talk about? The answer is that there is very much indeed - so much so that few expect the talking to be done in a matter of days. ''I do not believe that anybody is under any illusions that the talks will be a pushover,'' said Governor Mr Chris Patten, soon before leaving London to return to the territory yesterday. Perhaps he was thinking of the four-week standoff over China's insistence that Secretary for Constitutional Affairs Mr Michael Sze Cho-cheung and his Hongkong colleagues be described as ''experts and advisers'' - a deadlock only resolved when the two sides agreed to differ, and make no mention of the junior team members in the joint communique announcing the talks. More likely he was thinking of the difficult discussions ahead. For, now that China has taken the plunge and agreed to talks, it is generally felt they will be allowed to break down after only a few days. Although angry scenes are inevitable on Thursday, as Beijing takes Britain through its alleged violations of the Joint Declaration, Basic Law, and seven diplomatic exchanges, it is nonetheless expected that when discussions adjourn at the weekend they will be followed by a second round of talks, a fortnight or so later. Only then are the two sides likely to get down to serious negotiations over the three key issues at stake: the ''through-train'', and Mr Patten's two most controversial proposals, for a more democratic Election Committee and functional constituencies. And it is the question of a post-'97 through-train that has rapidly emerged over the past few days as the major stumbling block, with the local leftist press yesterday accusing London of turning it into an obstacle to talks. For China, the issue has already been settled by the Basic Law, which states a Beijing-appointed Preparatory Committee will decide who is allowed to remain on the through-train no earlier than 1996, based on its interpretation of which legislators ''uphold'' the future mini-constitution. But the British side is expected to tell its mainland counterparts this week that it does not see this as good enough. Officials say there is little point in making concessions on the 1995 polls unless legislators are guaranteed a seat across the transition in return, and Mr Sze told a closed-door briefing of councillors on Friday his team would cite the seven diplomaticexchanges in support of their demands on the through-train. It is recognised those who fail to uphold the Basic Law, perhaps by continuing to call for the overthrow of the Chinese Government, as Mr Martin Lee Chu-ming and other United Democrats have done in the past, can expect to lose their seats, and that the Preparatory Committee does have a role to play. Instead, all Britain will be demanding is the Chinese provide a precise definition of who will be allowed to remain on the through-train, before the 1995 polls, so candidates and voters know in advance what the rules are. And with a local pro-Beijing group scheduled to unveil tomorrow a possible formula for providing that definition, the gap between the two sides on the issue may yet prove bridgeable. For the Democratic Alliance for Betterment of Hongkong (DAB) is expected to propose the National People's Congress Standing Committee promulgate the requirements that through-train riders must meet, and the Hongkong Government acknowledge them in a memorandum, in a potential compromise already being welcomed by some within the administration. On the other two issues expected to generate the most heat at the negotiating table, both sides are also preparing fallback positions that will bring them closer to finding common ground. While China has yet to spell out its position on the Election Committee that will choose 10 legislators in 1995, it is expected to insist on the format mentioned in the seven letters between the two country's foreign ministers, and spelt out in detail inthe Basic Law. That calls for a four-sector body, with membership divided equally between politicians, professionals, businessmen and grassroots interests. But it was reported yesterday that, providing such a format was followed, Beijing was prepared to be flexible on whether its members were elected or appointed, in a stance which holds out some hope of moving towards an agreement on the issue. FOR the British side is prepared to scrap the Governor's blueprint for an Election Committee composed entirely of district board members, providing some other model can be found under which all of the up to 800 members are elected. Once again the DAB's proposal is being hailed as a potential basis for a compromise, since it sets out a formula for a four-sector body at least half of which would be elected, although the Government cautions it still does not consider this democraticenough. Similarly, with functional constituencies, where the Patten package envisages a radical departure from the past, with each of the nine new seats elected by large sectors of the working population, rather than the smaller groups and individual professionsused for the current 21 seats, it is also accepted the administration might have to revert to something akin to the existing system as part of a compromise deal with Beijing. Suggestions have already been put forward for allocating some of the new seats to trade unions, many of which are pro-China, or scaling down the size of the electorate from Mr Patten's proposed 2.7 million, so long as it remains larger and more democratic than the present functional constituency system. On the Chinese side, it is now being indicated that, providing Britain reverts to the previous format for functional seats, Beijing is prepared to bargain over how they are elected. There are also suggestions China will not push too hard for its favoured voting model, the multi-seat single vote system that was so disastrously proposed by the former Co-operative Resources Centre last summer, since it does not form part of any previous agreements between the two governments. But other issues are also expected to bog down the negotiations. Beijing opposes the abolition of all appointed members of district boards, arguing for a more gradual approach. All these complex problems mean that, even after two rounds of talks, it may still be impossible to tell whether there is any prospect of eventually reaching agreement. Instead that judgement is more likely to be made after three rounds of talks, in late May, when the Hongkong Government faces it final deadline for submitting the electoral legislation to Legco in time for it to be passed before the July 21 summer recess. That was something the Governor previously insisted was absolutely vital, if proper arrangements are to be in place in time for the 1994 and 1995 polls. But already the back-sliding has begun, with Mr Patten declining to reiterate that condition. ''If the talks are making good progress I do not believe that anyone in the Legislative Council would want to be legislating in parallel at the same time,'' he said in London. Elsewhere it was privately conceded that if negotiations do go well the Government would have no choice but to put back still further the introduction of the legislation, even if that means it can not be enacted until autumn. Either way, this means that all eyes will be on the clock when negotiations begin in Beijing on Thursday. How the sides ca do deals The Legco through-train: Britain wants councillors elected in 1995 to serve until 1999. China insists on being able to remove those it dislikes in 1997. Solution: China to set out detailed conditions on who can ride the through-train before the 1995 polls, and explain how Legco members will be asked to ''uphold'' the Basic Law. The Election Committee: Patten's proposal: An election committee, to elect 10 Legco seats, composed of District Board members. China says this violates previous agreements. Solution: A committee made up of politicians, professionals, businessmen, and grassroots interests, with all or most of them elected. Functional Constituencies: Patten's proposal: Nine new seats, with every working person eligible to vote. Against previous agreements, says China. Solution: A scaling down of the numbers allowed to vote in each constituency, but more representative than the existing narrow system. Possible inclusion of seats for pro-China trade unions.