In this case, lubrication is not the issue.
As you can see from the first photo I posted the other day there are 4 x ports on the mixing valve - so clockwise from the top, water to the pump, water from the return manifold, water from the boiler and finally, return water to the boiler.
My problem is that when I initially dismantled the valve the plastic housing (final two photos) came out with the piston and the spring and it is crucial to the valve operability that it goes back in the correct way.
Unfortunately for me, while I was chasing the spring around the floor I never noticed which way the wretched housing came out, hence my present predicament!!