Well that was fun. Not helped by the fact someone had clearly changed the o-rings in the past (access hole already cut out for screwdriver), but they'd cross-threaded the screw in to the clamping block.
After replacing the screw and chasing the threads out, life got much easier.
Next problem - purging air from the system. About 10l coolant out, 6l back in. Off for a run round some hills and corners now to slosh it around a bit.