Its called 'Throttle Hang' and is easily fixed by placing a cooper cap in the pipe that goes from the bellows to the Idle COntrol Valve. This cap needs to be drilled with a hole between 3 and 7mm to allow the air through. I would start with a 4mm hole and increase it until the revs drop freely.

The caps themselves cost around 17p from any good diy place and are the ones used for blanking off copper water pipes.

Any other problems pop along to www.southeastcarclub.com and I will be able to help.