Hi all, bit of an update, while poking around in the cupboard with the cylinder and valves I noticed a gate type bypass valve above the cylinder return pipe and before the 2 port motorised valves which was fully open. In other words a large proportion of flow from the boiler would have been going through the pump and then straight back to the boiler as the path of least resistance while only some would have gone through the motorised valve to the rads. I've shut this now bar the last 1/4 turn and my rads have never been hotter, they are hotter now than the fires of hell. I suppose I have some more balancing to do!