As FitBloke says Watford Met is the closest station, but I'm advised that there are works on the line around Harrow, so you should expect some delays on that route, but still a good option from Central London.
The alternative is Watford Junction which is a mile from the start/race h/q and offers fast trains from Euston and Milton Keynes etc.
It is a rule of all Road Races that numbers are not transferable. The most obvious reason is what happens in the event of an injury or incident? I'm sure you will all remember the terrible tragedies at the GNR a couple of years ago! Or - you've stated on your application form that you have no allergies, this is recorded on our database in case of emergencies, but the person who takes your number does have! All unlikely I know but it could happen and lead to real problems, please don't do it.
We have just found out that Watford Met line station will be closed, trains from London will terminate at Northwood and from there a bus service will operate. This will almost certainly lead to substantial delays, anyone coming from London should consider the normally very good service from Euston to Watford Junction with just a 10 minute walk to the Colosseum.
Apologies about the delay in getting race packs out to you all, initially there was a delay with the printers unable to produce the race booklet before Christmas then an error in the printing delayed it a few more days, we've used the same company for the last 20 odd years and this is the 1st time there has been a problem, although I know that's not what you want to hear. Approximately 1600-1700 race packs have now gone out with the remainder to be posted by Monday latest.
Andy, try The Moat House (0871 3308233) The White House (01923 237316)Bedford Arms (01923 440047) and Travelstop Inn (01923 237316) they're all within a 5 minute jog of the start/finish. Not recommending any of them as being family friendly so suggest you check they're websites. Or if you've got loadsamoney The Grove (01923 807807) is excellent and the half course goes very close, you're wife will love the spa.