SINCE Chris Patten left for the Cabinet summit on Hongkong policy, the signals from London have been perplexing. Mr Patten has stressed his determination to ''get on with things ourselves'' if there is no agreement with China. But he also has appearedto offer an unexpected flexibility on the through-train, the notion that those elected to Legco in 1995 will retain their seats after 1997. Yet Hongkong had been led to believe the through-train was a principle on which he would not give way. Douglas Hurd, the Foreign Secretary, added to the confusion by announcing he would visit Beijing next week for talks with his Chinese counterpart, Qian Qichen. Yet he said he would be going not to negotiate but ''to set the parameters''. It is hard to see in Mr Patten's statement on the through-train a reaffirmation of British resolve in the dispute over the territory's electoral arrangements. Yet it is no backdown. The Governor's remark that not all legislators in 1995 must have a guarantee that they can travel through to 1999, has to be seen in the light of a later clarification - that they must be able to decide for themselves whether they wish to stay on. The choice would be on the basis of objective and predictable criteria, rather than the whim of the Special Administrative Region Preparatory Committee, the body which will set up the new government of Hongkong. But Mr Patten is running a risk in demanding ''objective'' criteria for the continuation of legislators' terms of office. The idea is seductive, for it seems to offer guarantees. But it is a demand which the Chinese find hard to meet: on their reading of the Basic Law, only the Preparatory Committee can decide who rides the through-train. The trap might be that the only possible deal involves conniving at setting restrictions. It is essential, however, for the credibility of the 1995 elections that there be no agreement which lets China throw liberals out of Legco - not without fear of local and international condemnation. Provided Britain negotiates this pitfall, the acknowledgement that not every passenger need purchase a through-ticket may be seen as a small but useful opening to the Chinese, a sign of British flexibility and, therefore, their ''sincerity'' in trying toresolve the dispute. Whether the London meeting turns out to have been a watershed may depend more on the mood in which Mr Hurd goes to Beijing than on the ''parameters'' he sets. China needs to be convinced that Britain is sincere and flexible. But it should also be persuaded that, despite the give and take of negotiations, Britain remains committed to the principle that the elections must be free, fair and acceptable to the Hongkong people.