The guy linked to by
@Claptrap (Gary Hall) is a recognised Drivelodge Elevating Roof fitter who has been used by amongst others iirc,
@george shacklady and I believe Jap Auto Agent (where I got my E51 from, complete with Drivelodge roof).
Coincidentaly or not, Paul at Jap Auto Agent offers swivelling seat bases at £350 each and again, iirc, George Shacklady has used them in his latest conversion.
Some things to consider when going the swivel seat base route:
1. Some look very agricultural - not pleasing to the eye at all imho
2. Most will raise the overall seat height, so this may look odd is only doing one seat
3. If doing the driver seat, I would check to see if the OEM electrical seat adjustment is kept. I suspect not which may mean the forward/backward seat position for driving is in courser steps than that offered by the OEM system
4. If going for a side conversion too, you will need to sacrifice some side furniture space to allow the driver seat to come far enough rearward to clear the steering wheel on swivelling
5. And does the swivel mechanism allow the seat to be locked at different angles rather than just plain foward/rearward?
6. Finally, be confident in the mechanical strength of the swivel base - it's not likely to be M1 pull tested so you'll need to be confident in the structural strength of the base.
As mentioned above, I fashioned my own - see link
here. Three years on it has been trouble free and cost me £20 for some welding, otherwise all OEM fitting and anchor points were used.