Uh, Heseltine was on the backbenches from '86 onwards. (early January '86 at that)
I'm of a rather deterministic mindset in respect of the poll tax. Something like it was probably going to come along sooner or later. The idea had it's genesis in at least the early eighties, probably earlier. (Although it's worth pointing out that the first time it was considered as a serious policy option, in the early 80's it was reportedly laughed at by the committee examining it.) People think of it as a '89-'90 phenomenon, but it was already in train before the '87 election.
Maybe if you had a very determined, sensible, loyal Environment Secretary, who really had her ear, in place of Baker, Ridley etc maybe he would have been able to scupper it. But I can't really think of such a person. (Casting around randomly, but anyone know what Gow's views on the poll tax were?)