If it feels better with more fuel in it, then it won't surprise me if the fuel filter has not been changed. It is something you should do apparently every 30000 or 50000 miles or something stupid like that but I done mine every two years or about 12000 miles in my case they are not expensive so why wouldn't you change it. Not sure if the ST170 is different due to its different fuel system but on a normal MK1 is it located at the rear of the car near the OSR wheel.

P0420 is the normal cat efficiency message, I got that quite a bit on my decatted MK1. Usually it is lambda sensors that go first before the cat and MK1 Focuses commonly eat lambda sensors. You can pick them up off ebay pretty cheaply they are Bosch items and are fitted to Fiestas and Pumas and KAs and also all other petrol Focuses. IIRC they are all the same including that of the ST170. Green plug and wire I think is number 2, number 1 is blue but again same sensor just different attachment I think at one point I had two green ones running. To check if your cat is knackered, get someone to rev the car whilst having your hand behind the exhaust and see how it feels, if it is blowing reasonably well then I'd say it isn't a knackered cat. If you have an IR temp gun, get the car in the air whilst it is up to temperature and move it along the body of the cat and have a look at the readings. They should be fairly consistent. If they suddenly drop somewhere along the cat then you have a blockage.

This does sound like to me a case of a clogged fuel filter. I wouldn't be surprised if it got left out as its underneath the car people can't check it.