I think I've finally solved my battery drain problem. I was previously measuring the Ignition Off Draw via the IOD fuse. With the radio connected it was drawing 0.87 amps reducing to 0.05 with the radio unplugged. I left the radio unplugged for over a week but the new battery was still going flat within 2 or 3 days. Today I decided to measure the current draw using a multi-meter connected between the battery + post and the battery + cable. With the ignition off and the car locked I was seeing a draw of 1.4 amps!
So with the multi-meter still connected I started removing each fuse in turn until I pulled the fuse for the Amp. The current draw dropped to 0.03 amp.
I don't understand why the amp is drawing current with the ignition off or why the power to the amp doesn't got through the IOD fuse.
So I'm thinking of treating myself to a new head unit for Christmas but I realise I'll have to figure out how to wire the speakers back to the new unit.