Okay, ive finally gotten the heater valve/pump unit out..
I took the unit apart, and guess what i found jamming open one of the valves?
ROCKS... lots of tiny little pebbles, the rough ones like you would put in a fish tank.

How the hell do you get rocks, not dirt, ROCKS into your coolant..

Should i put the unit back together having cleared the rocks out from it, and give it a good clean.

Also, is there some way of testing that the unit works whilst it is apart?
Can i spark the connections on a battery to see if it works?