There are three ways to change it, first get hold of a new Grunsford pump. turn both valves off at both sides of the pump, lossen the two big nuts that go on to the pump, some times come off ok other times they are very tight!
Second if you have the same pump ie same numbers, lossen the 4 allen studs on the pump head hit it with a hamer to break the seal lift off and fitt new one tighten the 4 studs wire up turn on and you are back in bussiness.
And some times they are right tight drain the whole system lossen the 22 mm nut on the valves and fitt a new pump and valves, refill.
erj
March 2010