The deputy political editor of ITV News has reported on six elections but 4 July will potentially be the most significant

In the basement of a huge London office block the clocks are set to 9.30pm: we have half an hour to go. Professor Colin Rallings takes a seat at a large white table in the centre of a vast room painted all in green, as I and a small group of colleagues gather around. The excitement is palpable as ITV’s elections expert Rallings holds up a piece of paper and starts reading out the results of the exit poll.

Upstairs, shrouded in secrecy with blanked-out windows, the all-important seat projection numbers are being carefully punched into graphics, spreadsheets heavy with data are uploaded, dozens of colleagues input the numbers – all part of the massive simulation that is the rehearsal for Thursday’s ITV News overnight election programme.