Byelection upsets make headlines, but they seldom change the world. Orpington 1962, Dudley 1968, Crosby 1982 and Greenwich 1983 – each evoked startling coverage but none of them had much echo in the general elections that followed.

Sometimes the impact of a contest that fails to produce any switch of constituency control matters just as much. When putting together the last edition of British Political Facts, we included a list of byelections where an unexpected retention produced major reverberations (as well as of other byelections where the unsuccessful challenger fared surprisingly well). The Press Lords’ 1931 failure in the St George’s division saved Baldwin’s leadership. Quintin Hogg’s hold in Oxford in October 1938 shaped the appeasement debate. Labour’s increased majority in Hull North in January 1966 led Wilson to call the 1966 general election. Labour’s totally unexpected survival in Grimsby in April 1977 buoyed up the beleaguered Callaghan government. The retention of Darlington in March 1983 enabled Michael Foot to survive to lead the Labour party into the impending general election.

But a byelection is only a byelection. Glenrothes shows that Scotland is not England. Between them the Liberal Democrats and the Conservatives secured only six per cent of the vote. This fall from 21 per cent in 2005 must be attributed to tactical voting and the Conservatives’ boast that they pushed the Lib Dems into fourth place must surely reflect a different level of realism about how to ensure the humiliation of the SNP.

Commentators talk about Fife as Gordon Brown’s own bailiewick. It is worth noting that in an equally neighbouring seat, Dunfermline West, Labour lost the 2006 byelection to the Lib Dems.

Journalists, as well as party activists, do not come well out of the byelection. No one seems to have foreseen the turnout (Greenwich in 1987 provides the last example of by-election turnout falling by as little as three per cent from the previous general election). No one seems to have envisaged the increase in the Labour percentage of the vote let alone its comfortable 6739 majority. The SNP did indeed achieve a five per cent swing but everyone expected it to be so much greater.

The possibility that the SNP lost votes through being in power in Scotland and in Fife is more plausible. But does it imply anything for the national scene? Next June the Conservatives are expected to gain power in ever greater areas (Durham may be the only Labour-controlled county council left in England).

Over fifty years I have written many commentaries on byelections, offering words of comfort or condolence for each party in turn. I find myself as sceptical about my own past judgments as I am about most of the post-mortems since Glenrothes. But what is quite clear is that in the short run Glenrothes has provided a notable morale boost for Gordon Brown and his party and, still more, that it has changed the mood of Scottish politics.

For Labour the real comfort from Glenrothes is that everyone expected it to be so much worse. For the SNP it was a profound disappointment. But the lessons are primarily Scottish, and no one should forget that Labour still remains trailing behind the Conservatives in the UK polls.