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Without seeing the car, your problem is going to be quite difficult to identify.

Broken cats can cause an issue such as you are saying, but it is rare that they cause such a dramatic loss in power, unless it is completely blocked which is unlikely.  The cheapest way to check this is to drop the cat off the car and give it a little shake.  If you can hear a significant amount of loose debris inside then it could be your problem.

Other than that, has the car had an engine electronics diagnostic done on it?  This will cost you (price varies where you take it but an independent should be about £50), and will show if there are any sensors that show a fault.  Something like the air mass meter on the inlet side of the engine could be at fault, reducing the power.  It could also be running in "get you home mode" for some reason.

The best idea at the moment is to take the car to another garage and get a second opinion.  If you have any Ford specialists or a good independent garage in your area, give them a go.

Best of luck.

Agree - it could be the air flow sensor. Had this on a car recently (diesel however) and it felt like it was towing a trailer.

Thanks for that.  No - I don't believe that an engine electronics diagnostic has been carried out.  Would this eliminate any engine management sensors as the cause?  If so, this would appear to be my next step.  If no sensor problems are subsequently detected and there are no rattles detected in the cat, can any pressure checks be carried out on the exhaust or does it come down to replacing it to see if it makes a difference?