I'd assumed there was an airlock in the engine somewhere due to the coolant level rising in the expansion tank. A bleed tee is fitted in the top hose of the radiator, near the radiator. I unbolted the expansion tank and lifted it above the bleed tee and the radiator. On removing the plug from the top of the radiator found there was a lot of air at the top. Plug put back in when water rose to top. Removed bleed tee cap, no air in that bit, only coolant came out. Filled expansion tank to correct level, put cap back on, then lowered the tank down and bolted it back on. The expansion tank cap is brand new. After a couple of runs, tank level has gone up 25mm and doesn't seem to have gone back down even when the engine has cooled. Is there still an airlock somewhere, is it sucking in air somehow, other? What else to check?