Welcome Ginmoggie and that's a nice car you have bought.
To answer some of your questions:
1. Is it still possible to get an S&S air filter adaptor to enable fitment of different air filter covers in the UK?
Possibly, try Nobby (Spare manager) at Krazy Horse or see if Craig at M3WServices can help
2. I'm considering installing stage 1 pipes - is this considered an essential modification to dissipate heat?
Freer flowing exhaust pipes are probably the single best modification for your car. They will dissipate heat and make the engine run better and smoother. The Krazy Horse Stage 1 pipes are popular and I really like mine. Alternately Garage 56 pipes are maybe a little quieter but are much larger.
3. I have the heritage screen fitted and can see two gaps in the screen at the bottom of the centre alloy stem - what is their function?
I don't know about the Heritage screen but am sure someone will be along with an answer soon
4. Should I leave the quick release steering wheel retaining bolt in place which removes the quick release function?
YES YES YES. You will have the Traven quick release and there have been a few instances of the wheel not locating properly.
5. Can the front wheels be removed without removing the mudguards?
Yes, if the clearance is tight just let the tyres down first.
6. Where can I get longer seatbelt stems?
I got mine off eBay. Remember you need one with a 21mm Wide Tongue which is the smaller of the 2 standard sizes.
If you haven't already found it, the Alternative Parts List in the M3W Owners Resource sticky thread at the top of this forum is an invaluable source of parts info.
Good luck and I hope the weather soon improves so you can get the car on the road and enjoy it
