I want to fit 6 point harnesses to my car, and clearly need some holes in the seat base, has anyone else cut holes in MK fibreglass seats for harnesses ?

Not me, but I have cut plenty of other seats. It's important to weigh up the alignment of your mounting points and the holes in the seat. You'd be amazed how effectively a harness belt will tear through a fibreglass seat shell under an accident type load! A once tight harness can become very slack when it is no longer being being directed by the seat holes.

I drilled mine and joined the holes with the jigsaw from the rough side with a metal cutting blade. I put edging trim round the holes to stop rubbing.