When William Long retired as Prime Minister in 1987, after just over a decade in power, the hard, loyalist right of the Ulster Unionist Party saw this as their best opportunity to put a stop to the more liberal elements in the party leadership - but they had very few options for leadership, with Harry West being seen as too old and William Craig as too unstable - fortunately for them, the leadership contest took place in the aftermath of the 1987 Westminster election, in which Enoch Powell lost his South Down seat. Brian Faulkner resigned from his East Down constituency at Stormont to make room for him, and Powell was soon adopted as the loyalist candidate.
The result was a victory for Stratton Mills, who continued as Prime Minister until 1995, but he was cowed by the strength and fervour of the support for Powell, and gave way on more issues than his supporters had hoped. Meanwhile, Powell felt that his views had been given a vote of confidence, and he therefore did nothing to dissuade hard-liners from discussing a new party to take them back to the old days when everyone had it good apart from the Catholics, the EEC weren't poking their noses into harmless Ulster agricultural nefariousness, and there weren't as many immigrants in Belfast asking silly questions about why there had to be a sectarian divide at all.
The party that Powell eventually founded was not the party many of these men had wanted: he launched it together with PUP MP Clifford Smythe, and Conor Cruise O'Brien, the National Democrat MP for Belfast Falls, and its main policy seemed to be that Stormont ought to be abolished, along with the very idea of Northern Irish law, and Ulster's political parties should be pushed out by those of the mainland. Naturally, Stormont MPs from the Ulster Unionist Party were not in favour of these ideas, and even less in favour of the Southern socialist he was consorting with.
The British Unionist Party filled an Integrationist gap in the market on the Unionist side - with the UUP firmly in favour of the devolution which lined their pockets, and the fundamentalist oddballs of Paisley's party embarking down a path of Ulster nationalism, the BUP were reasonably confident that they had a constituency. As it turned out, they did, and it was called East Down. Clifford Smythe and Conor Cruise O'Brien predictably lost their seats in 1989, leaving Powell alone and very aware that his seven candidates had only made an impact in the most English areas of County Down.
One of these candidates was Robert McCartney, one of the few UUP men to follow Powell into his new outfit. He had come second in North Down, and had previously been a barrister and a prominent member of the Campaign for Equal Citizenship, an Integrationist organisation which had been integrated into the BUP - a hotly contested decision, as the CEC was deeply infiltrated by the British and Irish Communist Organisation. Indeed, Jeff Dudgeon, a BUP candidate and later Belfast City Councillor, was a member of BICO.
Having won a seat in the Commons, the BUP now had to enter the Senate. The Stormont Senate is elected indirectly by the members of the Commons, by a system of STV. Having only one vote, Enoch Powell was unlikely to be able to elect a Senator on his own, but a deal with members of the Ulster Liberals and NI Labour Party procured a favourable result - Robert McCartney was elected for two terms, and much was made of the fact that he was now a Senator, while Ian Paisley had been unable to keep his wife in the upper house.
The next election was three years later. The novelty had now worn off the BUP, and the UUP were keen to get rid of their most direct competitors: they piled all their resources into East Down to unseat Enoch Powell. This strategy saw mixed results. Powell's career was finished (a long way from where it started) but North Down, ignored by the Official Unionists, voted for McCartney.
As a member of the Commons, McCartney of course could no longer be a Senator (Senators serve for two Commons terms) so his resignation triggered a Senatorial by-election. By convention, but not by law, these are voted on under the AV system by the MPs of the same County as the outgoing Senator. The vast majority of County Down MPs were Ulster Unionists, so George Green defeated Enoch Powell on the first count, as expected.
McCartney's tenure in the Commons was just as undistinguished as his tenure in the Senate, and resulted in his defeat by the UUP in the very next election, when the UUP were under the refreshing new leadership of John Taylor - the Right of the UUP had finally taken back control, eight years after the Enoch Powell embarrassment.
The BUP continued under the leadership of Robert McCartney for a few more years, still with half a dozen Councillors at that point, but his manner in internal party affairs was alienating and diminished the electability of the party. In 2000, another Powell convert from the UUP, Jeffrey Donaldson, putsched him and he returned to the obscurity from whence he came.