On 18 October 1990, the Liberal Democrats achieved a seismic victory in the Eastbourne by-election, overturning a 16,923-vote Conservative majority. The result confirmed the political trouble facing Margaret Thatcher’s government and likely accelerated her defenestration as Tory leader.

The by-election was triggered by the assassination of the sitting Conservative MP, Ian Gow, a close personal friend and loyal confidante of the prime minister. Gow, who served as parliamentary private secretary (PPS) to Thatcher from 1979 to 1983, was killed by an Provisional Irish Republican Army (IRA) car bomb on 30 July 1990.

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The election campaign took place against a backdrop of protests over the poll tax, as well as emerging divisions within the Conservative Party – especially regarding Thatcher’s stance on European integration.

The Liberal Democrat candidate, David Bellotti, secured 23,415 votes (50.8%), winning Eastbourne with a majority of 4,550.

The result was a major political shock given the constituency’s long history as a Conservative stronghold. Significantly, the Liberal Democrats were not regarded as a serious threat at the time of the by-election.

Kenneth Baker, the Conservative Party chairman, responded: “Clearly the result is very disappointing and a very poor one for us.”

Conservative MP Ann Widdecombe, referring to Gow’s assassination, said the result meant the IRA would be “toasting their success”.

Widdecombe said: “Bellotti [the Liberal Democrat candidate] is the innocent beneficiary of murder. I suspect that last night as the Liberal Democrats were toasting their success, in its hideouts the IRA were doing the same thing.”

This victory was also notable in light of Thatcher’s mockery of the Lib Dems at the 1990 Conservative Party conference. Addressing Tory delegates on 12 October, Thatcher ridiculed the Liberal Democrats’ new logo: the so-called “bird of liberty”. She likened it to Monty Python’s famous “dead parrot” sketch.

Thatcher said: “I gather that during the last few days there have been some ill-natured jokes about their new symbol, a bird of some kind, adopted by the Liberal Democrats at Blackpool. Politics is a serious business, and one should not lower the tone unduly.

“So I will say only this of the Liberal Democrat symbol and of the party it symbolises: this is an ex-parrot.

“It is not merely stunned. It has ceased to be, expired and gone to meet its maker. It is a parrot no more. It has rung down the curtain and joined the choir invisible. This is a late parrot.”

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The “dead parrot” line was also a reference to a botched policy document that preceded the Liberal-Social Democratic Party (SDP) merger in 1988. The widely derided document was withdrawn after it was leaked to the press; Liberal Party president Des Wilson labelled it “as dead as John Cleese’s parrot”.

John Whittingdale, the political secretary to Thatcher at the time, later recalled having to explain the joke to the prime minister.

The result immediately influenced the long-running debate over Thatcher’s premiership. It reinforced the fears among Conservative MPs that she would not be able to win a fourth general election, having triumphed in 1979, 1983 and 1987.

The resignation of Geoffrey Howe on 1 November and his subsequent statement in the House of Commons are generally considered as the immediate triggers of Thatcher’s defenestration. The Eastbourne result helped shape the political environment into which Howe’s resignation was received.

Thatcher announced her resignation as prime minister on 22 November 1990.

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The by-election was also significant in shaping the development of the Liberal Democrats under Paddy Ashdown’s leadership.

The party, originally called the Social and Liberal Democrats (mockingly nicknamed “Salads”), had endured a difficult few years following the completion of its 1988 merger.

An electoral baptism for the new party came at the 1989 Richmond by-election. The “Salads” won 22.1% of the vote – behind the continuity SDP candidate on 32.2%, and the Conservative candidate, William Hague, on 37.2%.

The party also failed to return any MEPs in the June 1989 European parliament election, recording just 6.4% of the vote. It was beaten to third place by the Greens.

After the Eastbourne result, however, the Liberal Democrats secured further by-election victories in Ribble Valley (March 1991) and Kincardine and Deeside (November 1991).

The Liberal Democrats won 20 seats on 17.8% of the vote at the 1992 general election.

In the 1992-1997 parliament, Ashdown signalled the end of his party’s “equidistance” between the two main parties, indicating that he would be prepared to work with Labour to defeat the Conservatives.

The official Liberal Democrat website refers to the Eastbourne by-election as a significant moment in the party’s “recovery’.