Dammit Jon, i wish you had done this research in time for me to use the advice lol.
My experiences:
On South West Trains you are meant to reserve your bikes, (£3 on the day of the train, £1 in advance, at some stations they simply charge you £1 whatever), (Although some of the guards are nice and dont mind if you havent reserved and you just say you didnt know you had to).
Alphaline trains, if you dont have a reservation they can get shitty, one time they wouldnt let us on the train because we hadent reserved, then they put a food cart in the bike storrage bit :-
First Great Western, i had my first experience taking bikes on a great western train yesterday. So far this is the best service for bikes i have used! You have to put your bike in the guards compartment (If you have reserved far enough in advance they even put a reserved sign on a place on the bike rack reserving it for you). Even if you have not reserved, there is room for at least 6 bikes in the guards compartment so you are pretty much going to get on for definate! The guards did not ask to see our reservations because there was loads of room.
Sometimes even if you do pay and reserve your bike on a train and that train is busy, they sometimes wont let you on that train. If you have this problem plead with them lol and say summit like "We have already been pissed around, we HAVE to get this train" and they sometimes help you find/make room.
The best way to use trains with bikes is to buy your tickets, make bike reservations and plan your routes a day or so in advance...
"Tic"
PS Jon, can you explain these young persons travel cards a bit more? How long do they last what you get for the money etc...