Just an update, as the advance cheapies became available I have now booked Bolton>Euston....St Pancras>Gatwick single with a old codgers pass, first class MAN-EUS and the total came to around £44. Happy days!
Says there's no first class available on the BON-Picc leg (that's OK, knew that) or on the StP-Gatwick leg - thought there was but reading through it appears to be declassified.
Anyhow, brilliant price, chuffed to bits, more spare cash for a beer or ten in Gibraltar!!
However I have to stick to the trains given BUT I'm wondering does that apply to the local bits (BON-PICC) where there's no advance fare and to the final StP - GAT leg? I'm due on the 0954 from Bolton but wouldn't mind taking an earlier one (a) to avoid missing a connection - but the 20' should be fine and more importantly (b) a bit of time to grab a coffee and biscuit in the 1st lounge! I do know the Virgin 1035 is written in stone unless the first leg's late.
(I haven't seen the local ticket yet to see if it gives a specific train).