Having had the belts out of my M3 for servicing, and more recently just fitted inertia reel belts to my E-type (from the above referenced quickfitsbs.com) I would say it shouldn't be difficult to find an alternative to the MMC supplied items as there really is nothing special about them. The reel unit I used on my E-type was very similar in size and mounting to the ones used in the M3W, just different buckle arrangements.
It would be awesome if someone in the U.K. would work with the good folks at quickfits or similar to define a specification. Any seatbelt used in a machine with no weather protection will not live as long as one in a more conventional car, and with the earliest machines now 9 years old, a supplier should be able to shift quite a few sets.
