It's a 48 spline column (on my 64 plate 110)
But I'm sorry to disappoint- I had the OEM wheel off yesterday to count the splines and there's a new sensor appeared as part of the DSC electronics.
This sensor interfaces with the back of the original wheel (not the steering column unfortunately) and thus means you can't use any of the existing bosses that are out there.
I phoned around the houses yesterday and today:
Devon4x4: hadn't heard of the problem, and went to look at one of their mechanics trucks, and were stumped.
Mud UK (motolita stockists): hadn't heard of the problem, but are going to get back to me when they get their new 64 plate truck in next week.
Twisted Performance: knew about the problem, and are making a boss that fits, but aren't planning to release it on general sale (only for sale if you send your vehicle for a full upgrade).
Overfinch (a little out of my league but I thought I'd try): do a wheel that will fit directly- but it's £997 before VAT
Land Rover do a Momo steering wheel as an option on the Xtech/Fire/Ice specials but won't sell it aftermarket.
SO! It seems we're stuck for the moment...