Figured out how to secure the mast at the partners. The rear chock now is ornamented by two bits of aluminium:

• A plate on the bottom at the back to stop the back rising when the chock is pulled towards the mast;
• A lashing point on the top of the block.

In use it works like this:

This is quick and easy to set up and very secure. I can lift the bows of the boat by pulling on a line to the mast top without any sign of the mast or chocks moving. As with all these things, once you figure out how to solve the problem it is easy and obvious.