A better PoD would have been "what if the Unions had bit their lips in winter 1978?". If Callaghan could have pointed to a successful resolution of industrial disputes without seeming in hock to the unions, the big dip that happened after the Winter of Discontent could have been avoided.
To have Unions act like that you'd need a fairly big change though. After all large strikes had been successful in the past few years, why on earth would they hold back in '78? What did Callaghan have to offer?
If Labour don't try and keep wage increases low then inflation comes roaring back and Callaghan knows he ends up back at the IMF begging for more money, so he can't promise big wage increase after the election (well he can, but he'd be lying through his teeth). Equally as the unions had seen off the last Tory Prime Minister, and there was nothing to indicate the next one would be any tougher, the threat 'what if the Tories got in' wasn't going to cut it.
I can't see a POD where the unions are more open to reason but the same politicians still end up in the same mess.