The TV debate invites have been delivered - it's now up to the political leaders to accept or refuse

Invitations to participate in TV debates have been sent to the four main political parties. Credit: PA Wire

When history was made at 8.30pm just four weeks before the 2010 election, 10 million people tuned in to ITV to watch the first of the leaders' debates.

Two more would follow, on Sky and BBC1, and the debates reached a total of 22 million viewers.

The debates gave us 'Clegg-mania' and 'I agree with Nick' (a term still frequently used at the end of this Parliament).

So successful were they - voters, the broadcasters and most politicians presumed they had become a fixed part of the election calendar.

Not so.

All the main party leaders agree 'in principle' that the TV debates should happen.

But they have disagreed on the format, the timings and the number of leaders who should take part.

A repetition of the '3-3-3' format (three debates, over three weeks, with three leaders) was the most simple option but some of the parties disagreed.

Why should they happen once a week in the run up to Election Day now we have fixed-term Parliaments (remember Prime Ministers used to chose the election date about 5 weeks beforehand)?

Shouldn't one of the debates be held between the two leaders' most likely to be Prime Minister?

What about UKIP leader Nigel Farage whose party consistently polls higher than the Lib Dems?

So the proposal from BBC, ITV, Channel 4 and Sky News is for three debates, over 6 weeks with 4 leaders then 3 then 2.The invite has been delivered. It's now up to the political leaders to accept or refuse.