Rover 25 car reving up

i would like to know more, like has it been getting progressively worse day-by-day, or did it just go bad one day?
had you been working around the choke/throttle assembly before it got bad?
has your fuel consumption increased?

most likely:
1.lambda oxygen sensor faulty ; the engine needs proportionally less O2 when the engine is ticking over to when it is under load (in gear and driving) but the sensor is not detecting the change in oxygen. If this is the case, you might also find that there is some drive from the engine when it is idling (ie. foot off the accellerator, but in gear).(about £30 - £40 + labour)
2.there may be a leak in the inlet manifold (usually gasket), in which case, the O2 sensor is never going to feed back to the throttle enough to compensate for the extra O2.(buttons for the gasket, but may require some disassembly, dont know enough about the rover 25, but it might be a couple of hours or more labour)
3.idle control valve may be faulty or just a bit "sticky", so the amount of air being fed in is too much. (can often be fixed by spraying carb cleaner down the right hole in the throttle)
Hope this helps.

This happened all of a sudden, as i dont know much about cars i have done no work on the car and it is mainly used to get to and from work.
It does not do this all the time and only started as the bad weather came, normally tends to happen first thing on the morning.

lambda sensor more likely, it's just an on-off switchy thing (high O2 - on low O2 off or the other way round, whatever) so when it fails it tends to be sudden and may manifest itself in fuel mix faults like cyclical revving (vroom vroom vroom while ticking over). expect to pay about £140 at a garage outside london. I would get advice from the garage beforehand, don't mention anything about a 3rd party giving advice, just tell them the symptoms so they can make a more objective assesment.

Thanks for your advice.

Sounds like fault on engine stepper motor .