So took my Indy out for a quick blast at lunch time today and now I cannot select 4th or lower, I can switch between 5th and 6th.

3rd gear has been noisy for a while but I would shift straight past it and there was no metal in the oil when I changed it recently. Today I pushed hard and it made a horrid grinding noisy before getting into 4th and again into 5th. After that I couldn't select lower than 5th and just made it home.

What do I need to replace? Selector forks and drum? Something else?

It will need a full strip down and inspection,so pointless buying any parts until it's been inspected imo

I think that will be best, just wanted to start working out the damage to my wallet if I could!

I know ab performance have got a large quantity of 954 engines/spares if you do need anything.

Thanks, that is great to know, that way I'm not left with risky purchases off eBay, I'd much rather buy off somebody I can trust to supply quality parts.