The car keeps revving itself up while the engine is running (driving or stationary). The revs die down when the accelerator is dipped. Does anyone have any idea what the problem could be?

This could be a number of things,the only way to narrow the list is to have it 'code read'.Practically all garages are equipped to do this now,and should cost no more than £20-40 .Once you have the relevant fault codes you can repair it.

Idle control valve/Idle air valve/Stepper Motor.whatever Citroen call them.

Agree could be the idle control valve.

However, l cured a similar problem on a 1.6 VTR by replacing the engine management coolant temperature sensor (not to be confused with the temperature gauge sensor) The symptoms were similar, but not identical to yours, the engine revs would occasionally rise & fall automatically as the car was being driven or when it was idling. l discovered the faulty part by trial & error; when it came to detaching the cable from the management coolant temperature sensor, with the engine idling, the engine revs momentarily increased & fell back to idling automatically. l concluded that this sensor could well be the problem & replaced it at a cost of about £15 or so & bingo! problem solved.

The management coolant temperature sensor must have been breaking down intermittently thus fooling the ECU into believing the engine was suddenly cold, hence the ECU was increasing the revs (presumably thru the idle control valve) to compensate for what the faulty sensor was momentarily telling it. Assuming it is a similar set-up on ur 1.4 then the above could be worth investigating.