They do run very rich until up to temperature, after which they're OK (in good health).
MOT man should cane the ******** out of it until the cooling fan has cycled on and off a couple of times.
After that, richness may be a stuck open (or missing!) thermostat (engine will take forever to warm up), or faulty engine coolant temp sensor (blue coded sensor on the inlet mani). If that goes awol, the ECU can think the engine is cold when hot, so enriches the mixture regardless (equivalent of a car with a carb running with the choke stuck open).