Thanks everyone for your help. I turned up to a maintenance night, and I did need help: I was never going to recognise that the derailleur was bent. We couldn't see what the problem was, but no amount of adjusting the screws would allow it to move onto the lowest gears. The general scuffed nature of outside indicated it has had a rough life (yes, at my hands mostly).
Thanks for your help, I'm now derailleur shopping.
joan