Local government correspondent Rory Maclean explains why the Greater London Labour Party is in court - and what the case has to do with the London Mayoral election.

So who is taking who to court?

Six members of the trade union MSF are taking the Greater London Labour Party to court. They want to challenge a decision that they should not be allowed to take part in the electoral college which will decide who is Labour's official candidate to be Mayor of London.

Why isn't the union being allowed to take part in the vote?

The Greater London Labour party has a rule that unions cannot take part in annual conferences if they haven't paid their affiliation fees to the party by December of the preceding year. This is designed to stop people paying their subs just to get a vote at conference on a particular issue and not bothering to otherwise.

Isn't it to be expected that if the affiliation fee wasn't paid on time, the union wouldn't be able to participate - no pay, no say, to coin an apt phrase?

The MSF members argue that they did pay their affiliation fee although it was late and Labour was happy to take the money. They paid well before there was the prospect of an electoral college and therefore they should be allowed to take part.

Is it the MSF leadership that is bringing the case?

The union leadership is not backing the members directly. Some of them have actually been suspended because of the failure to pay the affiliation fee on time. However the union would be able to take part in the electoral college if they win the case.

Could the way MSF members are likely to vote, if they get the chance, be by any chance connected to Labour' decision to block them from doing so?

There are two views on this. Some of the union members who are bringing the case believe Labour just applied the rule blindly. However others think that it became convenient to do so when a number of unions likely to back Ken Livingstone were found not to have paid on time.

When will the result of the hearing be known? And if Labour loses the hearing, does that mean London MSF members get to vote in the contest after all?

It will be up to the Judges hearing the case to decide when the result will be known but they will be aware that if MSF is to take part in the electoral college it will want to ballot its members before the votes for the electoral college have to be in on 16 February.

Are there any other unions in the same position?

Part of RMT which has already decided to back Ken Livingstone has been excluded as well. If the MSF members win they will also bring a case. The train drivers union ASLEF is also in the same position.

Does any union want to vote for Frank Dobson? - Or Glenda Jackson?

It is understood the engineering union the AEEU will back Frank Dobson and there will be some votes for Glenda Jackson but so far its thought Ken Livingstone has the lead in the union section of the electoral college.

So, how will all this affect Labour's mayoral contest?

Frank Dobson dominates the third of the electoral college made up of MP's and MEP's. Ken Livingstone dominates the third made up of Trade Union block votes. That means the decision will finally rest with Labour Party members in London who make up the final third of the college