Blue tits nesting

Hi JP. Welcome to this mad family.  As has been said everyone is always willing to help if any advice is needed.  We all used to post on another forum before this one came into being and the people that ran that forum had exactly what you are describing.  They had one male and two females in the same box and had a mountain of eggs between them.  After a bit of bartering with the females they settled down and both tried to incubate the eggs.  I cannot remember how many actually hatched but I know it was a good amount, so can be done. You are right that it is somewhat later than some, but shows it is still possible for them to get going.  Good luck, and please keep us informed as to the outcome.  There are links to lots of advice on this forum, including the uploading of photos etc - and if I can manage it I am sure you will.  
Sue