In the confined space available I managed to tighten this nut up a little more by hand with a 24mm open ended spanner that I sawed in half. I guessed at the torque - this should be 60NM according to the service CD that I now have - in other words not especially tight. I could of course remove the camshaft etc and check the torque accurately with a torque wrench and 24mm socket, but with my limited expertise I am reluctant to do this.
Has anyone known one of these valve compensators to become completely loose and fall out, or are they designed not to, and is the 60NM torque setting critical?
I feel a little nervous about the engine with the way in which I have carried out this adjustment, and also thankful that I spotted this issue accidentally while replacing the valve cover gaskets.