There is just the one exit poll these days at British elections. MORI and NOP used to do seperate ones, now they carry it out jointly on behalf of BBC and ITN (and for the first time at this election, Sky), so all three channels will have the same one.
It is carried out at around 130 polling stations, and they conduct about 16,500 interviews. They try and use the same polling stations at each election (though changes in wards and polling districts sometimes make it impossible) so that direct changes from the previous election can be drawn. 107 polling stations will be the same ones as last time, with an extra 23 new ones, including some new ones in LD-v-Lab seats which were previously underrepresented. Unlike US exit polls there are no questions about why people voted, it’s just who they voted for.
Interviewers stop every nth person coming out the polling station, and give them a mock ballot paper to fill in, if someone refuses they are not replaced by another person. Every hour the papers are collected and phoned back to HQ, where they are weighted for differential response rates and crunched by people like John Curtice, Rob Ford, Clive Payne and Steve Fisher (if you were watching the BBC’s campaign show last night, Steve was the chap demolishing the myth of bad weather helping the Tories!). The first result comes out at 10pm on the dot, with a final projection at 11pm or so.
The aim of the exit poll is to predict the seat totals, not the share of the vote, and the team will try to work out if there are different shifts in support in different types of seat. The call is based on a probability of each seat going one way or the other, all summed up to make a seat total.
In terms of past accuracy, the exit poll last time got the Labour majority exactly right (though they were slightly off with Conservative and Lib Dem seats). Unless something goes terribly wrong, we should have a broad idea of the result a couple of minutes after 10 o’clock.