One ugly fact about elections in the UK is that they are decided in just a small number of the country's 650 seats.

Parties don't even have to be that close for it to matter far less how many votes you get than where you get them.

Anti-Brexit campaign group Best for Britain has some new data to illustrate this - and the importance of tactical voting.

They've calculated that as few as 40,704 tactical votes - moved between Final Say supporting parties - could prevent a Tory majority.

That's smaller than any single parliamentary consistency (the smallest of those is Wirral West, which has just over 55,000 people - the largest has 110,000 - the Isle of Wight).

That's because if the votes are in the right place they swing seats, whereas if they pile up in safe seats nothing happens - there is no effect on the election result.

Top 10 marginal seats to watch at the general election Show all 10 1 /10 Top 10 marginal seats to watch at the general election Top 10 marginal seats to watch at the general election North East Fife Currently held by the SNP with a majority of 2 Getty Top 10 marginal seats to watch at the general election Kensington Currently held by Labour with a majority of 20 Getty Top 10 marginal seats to watch at the general election Perth and North Perthshire Currently held by the SNP with a majority of 21 Getty Top 10 marginal seats to watch at the general election Dudley North Currently held by independent, formerly Labour, MP Ian Austin with a majority of 22 LivingInMediocrity Top 10 marginal seats to watch at the general election Newcastle-under-Lyme Currently held by Labour with a majority of 30 Derek Harper Top 10 marginal seats to watch at the general election Southampton Itchen Currently held by the Conservatives with a majority of 31 Rob Candish Top 10 marginal seats to watch at the general election Richmond Park Currently held by the Conservatives with a majority of 45 Robin Webster Top 10 marginal seats to watch at the general election Crewe and Nantwich Currently held by Labour with a majority of 48 Jaggery Top 10 marginal seats to watch at the general election Glasgow South West Currently held by the SNP with a majority of 60 Alec MacKinnon Top 10 marginal seats to watch at the general election Glasgow East Currently held by the SNP with a majority of 75 Christine Johnstone

They also think the Tory majority has halved from 82 to 40 seats since the end of November - thanks to shifts in the polls.

They recommend tactical voting in 36 marginal seats to swing the result - they each need fewer than 2,500 to take a seat off the Tories.

Supporters of a final say will wish them luck. If it were easy to mobilise voters like this, the election would be a foregone conclusion - but it's not. But what this illustrates above all is that we don't know what's going to happen on Thursday.