The BBC had a quasi-exit poll which showed Labour on course for a majority of around 120 or 130, and a lead of over 10% in the popular vote. If that had been accurate, then a Tory victory in 1978/79 would have been difficult even with the WoD. Much of the Labour government's problems in OTL came from its lack of a working majority, of course.
But let's not forget that Heath continued on after the narrow October loss for several months, and his defeat to Thatcher came as at least a mild shock. After a heavy defeat he'd have surely resigned immediately, possibly opening the way for a Keith Joseph leadership, assuming that he wouldn't have made the 'cycle of deprivation' speech in this scenario. Even putting that gaffe aside, Joseph came off as someone who made Alec Douglas-Home or Gordon Brown look like men-of-the-common-touch.