I've disconnected the phe and tried blasting water through it with a hose. There was a fair amount of black flakes that came out so I refitted it and now there boiler doesn't ignite. I can lift the little metal switch on what I think is the diverter and it'll fire up but the pin doesn't raise by itself. I've disconnected it again and tried to get more out but there wasn't any significant amount more so I've been to the shop to pick up some descaling, sludge removing stuff. The phe is now sitting in a box with the top pipe full of the liquid. I'll leave it for a couple of hours, rinse thoroughly and refit.
EDIT: Fernox F3 Cleaner
EDIT: Fernox F3 Cleaner