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Just thought I'd give an update and a drawing of the system.

House ch layout

Where it says 'other rads' these are upstairs ones.  Flow from heatstore is hidden by the heatstore return pipe.

The radiator marked 'bleed' rad is approximately in the right position and is the one (a) highest in the circuit and (b) furthest rad upstairs from the pump.  My plumber called me up yesterday to see how things were going as he has a free day tomorrow. We agreed that I'd try switching off the pump to see what happened.  Answer....an airlock when I started it up again.

I have a permanently connected drain cock in place of the bleed valve on the 'bleed' rad.  So connect up a hose and open the lockshield.  Some air bubbles and then nothing.  No more air.  No water.  Remember the system has been working fine and that radiator was hot and so has water in it.  So there's not enough head to push out any water/air in the pipes..  Unless ....unless  :idea:   you switch the pump off when you bleed, yes ?  In that case doesn't the pump act as a block to the flow...which explains why nothing comes out ?  But then surely the head in the tank is still acting to push the water out as it's acting back through the flow from the heat store, through the heat store and back up the return to that bleed rad...so it should vent.  You have to stick suction on the end of the hose to get it flowing again,whereupon air and water comes out of the hose/lockshield route.

This time was also different in that when I closed the lockshield and opened the flow on the rad, I got a lot of air out.  Very confusing