Hi Steve,
I would say that each of the responses are possibilities and ones that need investigating individually.
I have come across this sort of fault before and apart from the obvious brake fluid boiling and leaving an air pocket, there are other areas to look at as well.
Hoses, Servo, ABS (bleed fluid from the unions on the modulator block as this never gets replaced unless the ABS invokes), calipers, stuck pads in carrier, stuck caliper sliders, and probably a whole host of other areas but I would go through those first. Maybe the pads are the easiest to remove, clean and refit but if you have to prise any out then there's your problem.
Good luck and let us know how you get on mate, if we don't hear from you then.... (only joking!)
Cheers, Trevor