A DEADLINE by which Sino-British talks on political reform must end, with or without agreement, has been fixed by top British Government officials. The date remains a closely guarded secret, but sources in London have revealed that it lies in December, most likely around the middle of the month. They say that, after careful consultation with constitutional experts in Hong Kong, it has been agreed this is the latest that legal arrangements could be put in train, to allow the September 1994 district board elections to take place on time. Despite recent hints from Governor Chris Patten that his October policy address is a make-or-break point for the talks, well-informed sources insist this is not a real cut-off date. ''October is a false deadline, although we are trying to get the Chinese to believe it may exist,'' said one. ''There is an actual day beyond which we cannot go. But if it could be settled by December one way or the other, then the Governor could get through the arrangements he needs to put in place.'' British analysts dismiss Mr Patten's repeated insistence that he expects to give ''a pretty comprehensive report on where things stand'' to Legco on October 6 as simply negotiating tactics. They say it is designed to persuade Beijing that time is running out and that, if talks remain bogged down, the Governor may lose patience and go it alone as part of the policy address. Mr Patten stepped up these tactics in London last week. ''Of course we have a view on when we have to bring things to a close,'' he said, after meeting Foreign Secretary Douglas Hurd. ''I am not announcing an ultimatum or publicly setting out deadlines, but there is a point beyond which we cannot go if we are to have good, clean elections in Hong Kong.'' The administration is committed to publishing boundaries for the district board polls by the end of December, under the timetable laid down for the new Boundary and Election Commission. A senior Hong Kong official said yesterday subsidiary legislation for the elections would be put forward early in 1994, but denied this bore any relation to a deadline for the talks. He said the conclusion of the latest negotiations in time to avoid any delay in the district board polls depended on the outcome of the talks, and how much new legal drafting was needed. London sources also revealed the British side was now preparing to put fresh proposals at the ninth round of talks in Beijing on August 16. These will be in response to the Chinese proposals put forward during last month's negotiations, and - for the first time - differ from the Patten package put forward in last October's policy address. The Governor is understood to have been hard at work in recent weeks drawing up these alternative proposals. It is believed they will be broadly based on his original democracy blueprint, but with modifications to meet the objections Chinese negotiators have registered against any substantial increase in grassroots democracy and the size of the franchise. Meanwhile, Beijing was yesterday reported to have launched a fresh attack on Britain. In an interview in the latest edition of Bauhinia magazine, Chinese Foreign Minister Qian Qichen criticised Mr Hurd's recent comments on the increasing politicisation of Hong Kong, and said this raised doubts about London's commitment to past agreements. ''Someone said Hong Kong has become a political city, and that the situation has changed. So are the agreements between China and Britain still valid? This is a serious question of principle which we must treat seriously,'' he said. And Liberal Party leader Allen Lee Peng-fei moved yesterday to quash suggestions that he had paid a secret visit to Beijing to discuss political issues. ''I'm amazed at all the speculation,'' he said. ''We politicians should have private lives, so long as we keep within the confines of moral activity and legal activity.'' Mr Lee insisted he paid a one-day visit to the Chinese capital, purely to help close a business deal. ''Business is business and politics is politics, I would never mix the two,'' he said.