I've bought a ticket from Bangor now - £24 through to Shrewsbury on the 1508, changing at Chester & Crewe.
It was possible to get cheaper tickets, but not with bike reservations. Quite a few were sold out. Nikki, dear heart that she is, has offered to get a refund on her Pwllheli ticket and keep me company but I've said "Don't". It would cost her more from Bangor and there is still no guarantee of a bike reservation.
Although I'd planned two nights at Porth Colmon we could easly find another camp site a bit closer to the station(s) to reduce bike riding when there are trains to catch, which is never a bad idea.