Ann
are you absolutely sure the cat is the reason for "failure"?
What were the readings achieved?
I've had MOT test results which are quite frankly ludicrous, and either the machine was not working, there was a leak, it hadn't been zeroed or spanned etc etc.
I simply don't trust MOT grade equipment nor MOT grade operators. Having worked in industry with Horiba emissions analysers costing upwards of £1/4M (
) a time, you get to appreciate the sensitivity of this type of equipment and the importance of maintenance and calibration.
If it was only a fraction out of limits, I'd suggest doing a few basic maintenance checks, perhaps some new plugs, clean the connectors on the air and coolant thermistors, check the ignition timing (retard can tend to reduce HC at idle, you can always reset it after the test!) and ideally check the idle CO duty cycle as described in the manual (section 6E-59) and getting a test done at another station. That'll only cost a basic MOT, a lot less than £700 and worth a shot.
What sort of place was it you got this test done at?