United Nations, N.Y. — In a last-ditch effort to prevent bloodshed and to rescue the United Nations plan for a peaceful settlement between Britain and Argentina, UN Secretary-General Javier Perez de Cuellar personally telephoned Margaret Thatcher and Leopoldo Fortunato Galtieri, asking them for a 48-hour respite.
Reportedly he submitted to both some ideas of his own on how to narrow the gap between their positions on the Falklands dispute and how to implement the UN plan in a way that would not threaten their interests or compromise their principles.
This amounts to ''a gentle but firm banging of British and Argentine heads together,'' according to one source.
By all available evidence, most of the elements of a compromise between Britain and Argentina have been at hand for more than a week. Perez de Cuellar reportedly felt heartbroken at the idea that a package so carefully and patiently put together would come apart mainly:
* Because of domestic political considerations.
* For lack of political will on either side to walk the extra mile.
There is little hope, according to informed sources, that his ideas will find acceptance in London and in Buenos Aires at this stage. Mrs. Thatcher's initial response to the ideas Thursday was not seen as a good sign. ''The mutual mistrust between the two governments simply runs too deep,'' says a senior diplomat familiar with the process.
After the expected shoot-out between Argentina and Britain in the Falklands, the problem is likely to come back to the UN anyway--probably before the weekend. It is an open question whether it would be easier for the Secretary-General or for the Security Council to settle the dispute after national feelings and bitterness have been further aroused in both countries.