you start spending money replacing throttle bodies, you should try to reset the stepper motor.
The idle speed is controlled by the MEMS which in turn controls the Idle Air Control Valve (IACV or Stepper Motor), here's how it works...
The throttle body is basically a tube with a circular flap (butterfly) inside on a spindle, when your foot is off of the accelerator the flap is fully closed, but the engine still needs some air to run. This is achieved with a bypass circuit, basically a hose that runs from one side of the butterfly to the other with a valve in the middle (stepper motor) to control the air flow.
There are a number of potential causes for your problem Broon, ranging from something as simple as the footwell mat affecting the return of the accelerator pedal, a dodgy spring on the pedal, through a sticky throttle cable to, as has been mentioned above, a warped throttle body (causing the butterfly to not close fully - this extra air plus the air in the bypass circuit = high idle). But although the warped throttle body is reasonably common, there is another potential cause that is free to fix, so you should try this first.
Copied and pasted directly from Rob Bell's site...
Resetting the IACV
If the throttle body has been recently replaced, or you find that the idle speed is a little on the high side, it may be worth attempting to reset the throttle position sensor (TPS). This is readily achieved by following the following procedure:
Start engine and warm engine to its normal operating temperature (essential, as when cold, the engine requires some extra air/fuel - which on the MGF is managed by the idle air control valve: best not confuse the issue!)
Now turn off the ignition.
Switch on the ignition to position II without starting the engine (all the pre-start warning lights ought to be illuminated)
Depress the throttle pedal <5 times> to the carpet and fully release inside a period of 30 seconds or so.
Wait for about 15 seconds...
Switch off the ignition
The engine can now be re-started; the TPS has been re-set.
As i said, it may still turn out to be a warped throttle body anyway (and if it is it is an opportunity to get a 52mm Trophy one), but this is worth a try.