The most promising poll the Liberals had in this period was in July 1993 when MORI showed:
Labour 41%
Liberal Democrats 28%
Conservatives 27%
A result like that would be likely to produce a Labour majority of similar size to OTL but with the Conservatives and Liberal Democrats virtually tied at around 100 seats each, it's possible the Liberals would have very narrowly become the opposition. Assuming this result is possible we need the Conservatives damaged even further than OTL (revelation of the Currie affair is a favourite) but we also need Labour to drop in support as well. The best way to do this might be having debates in 1997, with a similar effect for Ashdown that Clegg enjoyed in OTL, have Paddy on top form with Blair being Good But Not Great and Major dreadful, also have it far closer to the actual election, perhaps a week or so, so there's no chance to change the narrative.
With this in place, the final result could come out something like:
Labour 415
Lib Dems 108
Conservatives 107
I wonder who Ashdown would have in his Shadow Cabinet...