I've priced up national express (on their website), mega bus (their website) and trains (in trainline website) and train seems the best because it's a tenner dearer in total for the 5 of us but the journey is nearly an hour shorter. £69.80 for my dates.
It sort of seems to good to be true though. Does anyone know if the trainline prices include travel from airport to train station (airport parkway) and if it's worth booking now rather than paying on the day? It looks like the tickets are the same price whenever and can be used on any train.