Hi Jim. Three main types of train in France: TGV – bikes carried on some of ’em but must be booked: Corail – Bikes carried, should be booked (possible to get away without booking, a bit like UK trains) TER – regional trains, bikes carried, no need to book. RailEurope can do the booking for you 0844 848 4078, you can’t book bikes online.
Eurostar carries bikes for £25 each – first book your train, then you ring EuroDespatch on +44 (0) 844 822 5822 and book the bikes on (I think you can book Paris – London at the London office, give them a ring to check before you book the train. Be patient, it takes them ages to answer the phone!) It actually works really well – just turn up at the Eurostar office with your bit of paper, hand the bikes over and you won’t see them until you get back to London and collect them. Although they say stuff about the bike not necessarily travelling on the same train as you, if you hand in the bike well in advance (what we did was drop it off the night before we were travelling, so we didn’t have to worry about them and could go off and get pissed bike-free) it will probably get to London before you. Although you can pay an extra fiver and they guarantee the bike will be on your train, it’s not worth it.
Hope that lot helps – anything else you need to know, get in touch!