revs/accelerator sticking

I've got a Rover 416si N reg, and over the past couple of weeks the revs
don't always drop back down to tickover when I stop at the lights or slow
down for corners etc.  I have to blip the accelerator up a bit to "clear"
it, and then the revs drop back down.
It's got worse this week, and I've had to change the way I slow down due to
not allways having engine braking to help slow down with.

Any ideas what might be causing the problem?

Thanks a lot.
Garry