Richard calling you a thick git was mild considering the first word i tried to use was censored , yes it was a little rude but justified considering you were being a sarcastic wally ( hope 'wally' isn't too rude for you

) but if i offended you I apologies
my point was well justified as max agreed, the symptoms he gave didn't make sense, so for any one to give an informed answer was going to be hard. he needed to give more details.
did he manually fill the header tank by sticking a hose directly into the H/T, or from the inlet at the front, perhaps the float valve in the H/T is stuck or the inlet filter from the fresh tank is blocked
he mentions descaling the machine, did he put descaler in the H/T and run the descaler in a circle through the machine back into the H/T this could have deposited loads of crap into the H/T that still need to be flushed out, Did he put the descaler in the fresh tank and feed it through the supply pump
does the pressure build up when he lets go of the trigger then go as he pulls the trigger.
there are lot of problems that need to be investigated, without additional information any reply would be a guess..... that is why I made my first post.