Clive,
You could try making up a makeshift jack from scraps around the workshop to press the pump out of the case. Take a short length of, say, 12mm studding, a nut and washer, and a suitable length of 16mm dia steel tube. Put the nut and washer on the studding, slide the tube up to the washer. Insert the jack between the oil pump and the top of the crankcase (a soft packer may be needed to prevent damage to the casting), apply a reasonable load and leave it for a while, then keep coming back to it and top up the load - eventually, the pump will budge. The postwar oil pumps were not provided with any easy means of extraction and if gasket cement like 'Osotite' (which sets solid) has been used in the past, the pump may be virtually welded into place. When replacing the pump, make sure all faces are flat and then reassemble with either a non setting gasket compound or better still just a smear of grease on the gaskets - then subsequent dismantling will not be a problem.
Regards,
Eddie.