Thanks for all the replies. There are too many for me to go through and quote, although I would say that ultraman was a little harsh about the RD placement

CNY - It seems unacceptable in going up to gears 4 and 5 - it seems to take a long time to go up (around 10 seconds), just constantly clicking away, sounding as though it's on the verge of eureka, but not quite getting there.

The cables have not long ago been replaced due to it bluntly refusing to change up unless I let out loads of extra cable, so I doubt it's that, although I have ridden in VERY heavy rain since those were done.

It's a basic Shimano Tourney RD, done about 2000 miles on it. No idea which way it's sprung unfortunately, but I went into an LBS today and will take it in there next week to see what they can do. I guess I can pay for that repair then

Thanks for all the help - over the next week I'll have a play with the cables.