Mountain Bike Tech Help!

I think the index might be damaged as even when I remove the cable from the rear derailer completely, it can still only click up and down once..

If you release the cable tension completely and pedal, does the chain fly down to the smallest sprocket? If it does, then this tells you the derailleur is moving okay and isn't seized.

Back off the high and low limit screws all the way and see if that helps.

And I've just seen your 2nd post about the suspected knackered shifter... I'm afraid if that's borged, then it isn't something that can be fixed quickly. But it's worth checking the cable routing into the shifter and that it isn't fouling the mechanism.

Thanks Ben, when I removed the cable, the chain did go all the way to the bottom, in fact as I had the limit screws really loose, it came off the cog, so that all seems as it should. I am now quite sure it's the shifter that's knackered. I'll try and settle the chain to sit somewhere in the middle of the cassette range so it can at least be usable tomorrow.