The ancient Romans considered Chester as strategically important as London. This week, it is arguably just as central to Labour and the Conservatives. 

People in this affluent northern city will vote in the first Westminster by-election since Boris Johnson departed, the pound crashed, and Rishi Sunak replaced Liz Truss as prime minister.

The contest was triggered by the resignation of Labour MP Chris Matheson for “serious sexual misconduct”.

But the result will be a crucial insight into how voters feel about the recent political and economic chaos, and which party is best placed to deal with the rising cost of living.

Labour has a majority of just over 6,000 votes – or 11% – in this constituency, and few political observers expect anything other than a win for Sir Keir Starmer’s party.

But Chester, with its mix of densely populated urban areas, historic sites and rural villages, is a microcosm of the challenges facing both Labour and the Tories.

Its relative wealth (despite pockets of deprivation) with house prices, wages and educational achievement all above the North West average means the Conservatives should do well here. And for most of the last century, they did.

It was Tony Blair’s appeal to the educated, aspirational, and affluent that broke that hold in 1997 – and despite a brief return to the Tory fold in 2010, it has been Labour since.

That Labour appeal to more educated voters is something we’ve seen more broadly in recent years – and saw again at May’s local elections. The party got a bigger swing from the Conservatives in areas with more university graduates.

It is costing the Conservatives dearly in seats they once held comfortably. Labour’s challenge is to inspire these voters without losing their less affluent base. It’s a balancing act the Lib Dems could disrupt in Chester if there’s no progressive pact.

In the years just before the Cameron-Clegg coalition, the Lib Dems polled at about 20% here. Having lost their deposit in 2017, the party’s candidate was the only one to increase their vote share in 2019.

So, how should we judge the parties’ performances?

If the opinion polls are correct, the 38.3% share the Conservatives won in 2019 would halve. Rishi Sunak’s ideal result would see the party maintain that 2019 share. A more realistic ambition is somewhere in between – a fall in vote but not by half.

For Sir Keir Starmer, the only way is up. Labour needs to increase its 49.6% share. A double-digit rise would reflect the party’s current support in the polls. Less than half that would return it to the 56.8% achieved in 2017 but could be considered disappointing for a by-election in these circumstances.

Much of this depends on which Chester voters, and how many of them, turn out. At the last general election, 72% of them did so – but turnout is invariably lower for a by-election.

Since 2000, in contests involving the main parties, it has fallen 23 points on average. So, we might expect about 49% to 52% of Chester’s electorate to cast a ballot.

It could be their last chance to do so in this constituency that has been returning MPs since 1545. One of the oldest in the UK, proposed boundary changes would split it and merge it with neighbouring areas creating two new seats, possibly as soon as next year.

If this is the last election in its current form, it’s already a record-breaking one. There are more candidates standing to become Chester’s MP than ever before – a total of nine.