What time period would he have left Labour?
If i where to hazard a guess (i'm not an expert on UK politics), If someone from the hard left like Tony Benn became party leader (as ASB as that sounds), then perhaps their would be an exodus of the Labour-Right (and even some moderates) to the Liberal Democrats, assuming there isn't complications in changing party (i'm sure there would be something preventing a sitting mp from casually switching party allegiances, he may have to stand down and re-contest his seat at the next election), maybe he is promised a prominent position in the party.
On a Blair lead Lib-Dem government, I mean after a certain amount of time i'd imagine, just like now, the hard-left's policies would not be so popular, also the Conservatives by the mid 90's where having a bad time, so its possible with no other real viable option, maybe in 97 or more likely, in 01 he becomes PM.
It sounds like it would take a lot of things changing in Labour, The Tories not taking advantage of that change and the Lib-Dems filling the void.