Quick update - that may help others in the future.
Removed steering column yesterday - managed to access the shear bolt with my dremel and a cutting disk to firstly grind a slot in the head of the bolt and then cut through the flange from the column itself that was secured by the bolt. This gave me enough wiggle on the column to release it from the shear bolt - which with the pressure then off, could be hand unscrewed!
A quick tap on the base of the column in the engine bay and the column was free and could be pulled into the cabin through the dashboard (which I did not have to remove at all). With the column free, it was evident the bottom bearing was very badly seized - it looks like a leaking clutch master cylinder had been leaking onto the shaft of the steering column and this had helped promote corrosion. Drifting off the bearing in its rubber mount showed the damage.
The column really needs to be replaced as even with 1200 grit paper and metal polish, some of the wear was still evident. Until I find another column though (and now I know it is fairly straighforward to replace - especially with normal bolts in the place of the shear bolts) - I reassembled the column and put it back. It is wonderfully free now to turn, although the column wear on the bearing does have a faint grinding sound...
Anyway - here is the rotten bearing after I had removed it from the column:
A real mess.