I recently broke off the thin plastic strip which holds the 6mm seat retaining spring clip in place, so I had to sort it out before the MOT. I believe a loose seat could well be an MOT failure.
I have to admit that the seat latch mod which Jon has posted was really good and addressed the rubbish original seat mounts. I was very tempted to go down this route, but didn't like not being able to lock my seat.
Therefore, I fabricated up some small stainless clips and riveted these to the seat allowing me to utilise the original seat mounting method.
I have fitted a small length of bicycle inner tube to the oil filler spigot to stop it chafing against the tank. Consequently, this has pushed the seat back slightly making the seat a tight fit. Therefore I added slots to my brackets and ground back the plastic on the seat to give a wider range of movement on the spring clip.
It took a bit of time to sort out, but it fits like a glove and the seat is solid.