THE heat is on at Diaoyutai. As soon as the British and Chinese negotiators take their seats at the scenic villa on Monday, the mainland hosts are likely to baffle the London side before any meaningful talks can proceed. Last Wednesday, Mr Lu Ping, the most senior official in charge of Hongkong affairs, told a visiting local youth delegation that the British side has to answer a question when negotiations resume: is it really serious about the talks. If not, the issues would remain unresolved. The warning came amid a storm of criticism from Beijing of British ''sincerity'' in the past fortnight. Chinese officials said they were furious that the British side kept ''pulling little tricks'' and ''causing new obstacles'' to the talks. This is in spite of what they claimed was their ''sincerity and goodwill'' to co-operate by giving the green light to the land sale plan, three franchise agreements and the resumption of Airport Committee talks. A mainland official said: ''They have gone too far. Those little tricks followed one after another. There is a limit to what we can tolerate.'' Beijing's attack targeted the passing of a bill on election boundaries and the setting up of an election commission by the Legislative Council just a few days before the fourth round of talks, which ended on May 29. Officials also vehemently criticised a reported statement made by the British minister in charge of Hongkong affairs, Alastair Goodlad, when meeting Legco members, that they could overturn agreements reached by the Chinese and British governments. Some Legco members later said Mr Goodlad had been misquoted. The strong response from Beijing reflected the depth of mistrust of Britain even though the two sides are about to hold the fifth round of talks. Some officials privately admitted that they could not rule out the possibility that Britain would still table the electoral bill to the current Legco session by claiming that the talks could not go on ''indefinitely.'' Remaining extremely cautious about the outcome this time, officials conceded that only slow progress had been made at the previous four rounds of negotiations. At most, differences between the two sides on how to interpret the seven exchanges between the two foreign ministers have only been reduced. Various sources said the two sides have only been able to reaffirm some points previously reached on the 1995 electoral arrangements, which ironically have never been considered as a real issue. ''As mutual trust has been broken, the Chinese side has demanded to go back to issues one after another and get a yes from Britain. That's why the talks have taken and will take a long time,'' a source said. In spite of rhetoric that Britain would re-negotiate with China on the number of directly elected seats in the future, sources said the two sides have reaffirmed that only 20 members will be elected by direct polls in 1995 in accordance with the provisions in the Basic Law. But the electoral methods, including the size of the constituency and the number of seats in each constituency, have yet to be discussed, as well as the composition of the Election Committee. Sources said both sides have had no major difference over retaining the existing 21 seats selected through functional constituencies in the 1995 legislature. How to allocate the nine more functional group seats remains unresolved. A source said the Chinese side had insisted that the principle of functional constituency elections laid down in the Government White Paper in 1988 should be maintained. It is understood that Beijing is strongly opposed to turning the indirectly-based functional group polls into direct elections with another name as proposed by Chris Patten. An official said: ''It is not difficult to reach an agreement on the allocation of the nine functional constituency seats. We will not go too far to ask for a seat for the NCNA. You can judge whether our demands are reasonable after they are made public.'' Although some ''concrete issues'' have been barely touched upon in the previous talks, various sources said the Chinese side had yet to respond to specific points with their own proposals, such as where the nine functional seats should go. The most contentious issue remains the through train. A source said the British side has insisted that there was nothing to be discussed if the members elected in the 1995 elections were not able to continue after 1997. ''I don't think the British have made any change on its position on the through train so far,'' he said, but added there were no concessions China could make on that issue. A mainland official said the power of the National People's Congress Standing Committee's Preparatory Committee to vet members was non-negotiable. ''We can discuss with them about the through train itself so that the structure of it in 1995 will conform with that of 1997. But whether the passengers on board can continue their journey is not a matter for negotiation,'' he said. It is understood that the contentious issue of the vetting procedure has been put aside, at least temporarily, as both sides tried to agree on the electoral details. The real issue remains so near yet so far from agreement.