In reply to Philip:
Sounds like you know what you're doing - Lord Derby has a slight large habit (the mature size is determined by both the rootstock and variety), and has good disease resistance to canker and scab, although prone to brown rot and has frost hardy flowers - so that on M26 seems a good choice. You're right to reject MM106, particulary as it would be prone to root rot on your soil.
I've not seen a containerised Lord Derby on M26 for sale, but since there are plenty of other apple trees in the area, you may not have to worry about pollination and could choose another variety -
http://www.mailordertrees.co.uk has a few M26 bush and half standards left.
http://www.orangepippin.com/ is good for researching varieties.
Autumn is supposed to be the best time to plant containerised trees so hope everything goes well.