Hello!
Stupid question alert - sorry 

I'm trying to book a train ticket for a few weeks time - 3 separate legs.
The train I want to get is saying seat booking is mandatory, but no seats left to reserve so unable to buy that ticket.
Train a couple of hours later has seat. Same price as previous train.
I think the T&C's are implying the ticket can be used on either train. Does anyone know? It's a super saver off peak open return.
I have friends who are booked on the earlier one, and when I went to book I couldn't! I'd like to travel with them if possible as it's a long journey.
I'm not a regular train goer so totally clueless about this sort of thing!