what's involved in removing the rear bulkhead?

depends if you want to remove it completely (csw style) or just remove the top half so that you can move the front seats back further.

removing it completely and fitting a csw type bar is a tub off job - probably a good 2 days work and 2 or 3 hundred quid for the bar.

fitting the MUD bar takes a couple of hours and costs about a hundred quid.

MUDSTUFF.CO.UK - Defender Bulkhead Removal Bar - Products - Have you got MUD? - Land Rover Solutions

done mine without removing the tub was a bastid of a job tho took me all day and a lot of jiggery pokery completely removed it and now have a tubular bar