From the Solent A7C Newsletter with many thanks

The following instructions describe the type "T" AC pump commonly fitted to the Seven, but can be applied to the similar "M" type fitted to earlier cars.

Remove the pump from engine and note the position of the inlet/outlets. Unscrew and remove the six screws which hold the two halves of the pump together. If the halves don't come apart readily hold the base and gently tap one of the unions in an upward direction with a screwdriver handle. Put the top half of the pump to one side.

The diaphragm which is retained in the lower half of the pump can now be removed as follows:-

Fit the new diaphragm as follows:- 

FINALLY, before starting the engine, turn it slowly on the starting handle to make sure it turns freely. Make at least three full turns. The reason for doing this is to check that the pump lever is correctly positioned and turning by hand will show this. If it is not correctly fitted the engine may jam before any lasting damage is caused.