1988..
To be fair, there were other events in 1987 which might have moved public opinion. The stock market crash in October and the big storm which ravaged the south might have enforced the perception that the Thatcherite economic miracle wasn't going to last or be built on firm foundations.
As Iain and others have argued, David Penhaligon would have been the obvious choice to lead the newly-merged Alliance Party and would, I think, have come over well on television as a more moderate voice beside Margaret Thatcher and Neil Kinnock.
It would be complex - in OTL, the vote share was, if 42-31-23 or thereabouts. A split of say 37-31-28 would make a difference in terms of seats but I don't know how much.