Head problem,I think ????

Hi to you all I'm new to land rover 90's and this is my first posting on the LRZ forum so please go easy on me ! The problem I have is my 1988 90 defender 2.5 TD is throwing all it's coolant out of the header tank when running, the local garage will not have anything to do with it, saying it's a piston gone !!! I can't see that's the problem as she runs as normal and can cover about 20 miles before the temp gauge starts to rise above normal, then if you remove the header cap there's a hell of a lot of pressure, bubbles ect. top the coolant back up and off you go again ! My little knowledge points me to the head gasket, can anyone help ? Also how difficult is a head gasket to renew is there a spot on the forum with instructions on " how to do it " ?
Thanks for your help in advance.
RUSS

dont bother with the td engine search out a decent 200 tdi to transplant in it will make a great difference in performance and consumption

Sounds like the classic symptoms of a head gasket. If it was a piston it would run like a dog (if at all) and I dont see how a holed or broken piston could pressurise the coolant.