Gary I suspect your MOT man may be taking the sliding seat regs a bit too literally. My MOT chappie said that it only applies to vehicles with sliding seat mechanisms to ensure they're in working order, and that fixed seats are fine.
Just point him to the first paragraph of Section 6.2 of the Car MOT Manual, which says
Original Design characteristics and specialised modifications e.g. to enable wheelchair access are to be accepted.
Obviously, fixed seats in a specialist vehicle are original design characteristics so must be accepted.
http://www.ukmot.com/manual/6.2/Seats-and-Doors