A rear silencer with o hole in it will invariably fail an emissons test on Lambda values. If the gas levels do not met the initial 30 second BET test then a full test must be carried out. A full test can mean that the engine will need to be run at a ‘fast idle speed’, the speed of which will depend on vehicle and engine type but would normally be between 3000 and 3500 RPM, for between six and ten minutes.
A vehicle, functioning correctly, should be able to regulate it’s engine temperature regardless of RPM even without being driven.
However, you should just ask to speak to the tester. Find out if the is an over-heating issue or the emissions failure was added to avoid the prolonged emissions test on a vehicle that was clearly going to fail.
To add the failure for this reason isn’t something VOSA would approve of so tread carefully if you want a straight answer.
A ’94 Clio only failing on a rear silencer leak is a very good result!