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I can reveal that Essex police have started approaching people who were at the seminar and dinner at the London School of Economics with Vicky Pryce on the night in contention, 12 March 2003.

One of her fellow participants has now confirmed this to Newsnight.

This development implies that, as I first suggested on Newsnight 11 days ago, the case revolves around whether Chris Huhne's wife can produce an "alibi" to show that she was at the LSE for most of the evening and therefore that she can't have been driving - or speeding - on the M11 that night, an offence for which she seems to have taken three points on her driving licence.

My revelation earlier today that the case revolves round a speeding offence around 11.20pm on the southern section of the M11 also has various interesting implications.

First, it seems very strongly to suggest that Huhne had indeed returned on the Ryanair flight from Strasbourg to Stansted, which was also first suggested by me on Newsnight at the start of last week.

Second, if Vicky Pryce had gone from the LSE to collect her husband that night, then the latest that she could realistically have left the dinner at the LSE would have been 9.45pm, if she drove up to Stansted.

That would allow 95 minutes to get to her car, drive the 33 miles to Stansted, park and collect her husband, and then drive back to the point where the speed camera clocked their car.

Third, if she drove from the LSE to Stansted, then presumably she would have had to leave the car in the vicinity of the LSE around Aldwych before the start of the seminar at 6.30pm.

In which case, it's hard to see how Vicky Pryce would have avoided the new London congestion charge, which had come into force just three weeks earlier, on 17 February 2003.

At that time the charge applied until 6.30pm in the congestion charge zone (it's 6pm nowadays). The LSE was certainly within the initial congestion charge zone covering central London.

It's possible that she collected her car from outside the congestion zone but that would have made her timetable even tighter.

Essex police will presumably want to check records of the Congestion Charge from 12 March 2003 to see if the Huhnes' car was clocked, and also the couple's credit card statements.

The other possibility, I suppose, is that Vicky Pryce went by tube and train from the LSE to Stansted and then drove her husband back to their London home in a car that was already parked at the airport. But that would probably have involved her leaving the dinner earlier than 9.45pm.

This evening, in accord with what I wrote here earlier, Vicky Pryce's solicitor Sarah Webb, issued a statement saying: "Vicky Pryce continues to co-operate with the police as Chris Huhne MP has indicated he does."

Chris Huhne denies that he asked his wife to take points for him on her driving licence.