sounds like it's running rich off boost, so pootling about will build up soot in the exhaust system that clears when you boost it and the exhaust gets a high airflow thru it.

get it somewhere where they have a emmisions tester (the local mot place) and get them to check it on idle. they will advise you what it should be. if it is running too rich, you can adjust the potentiometer on the ecu to bring it leaner.

be aware though, that runing rich off boost does not mean that it is also running rich on boost. the map may not suit the car correctly and someone may have adjusted the fuelling with the poti to make it rich enough on boost (the poti basically works as a fuel multiplier throughout the rev range).

there are numerous other problems that can cause it to be rich, such as defective coolant temperature sensor (the blue one at the back of the inlet side of the head), incorrect fuel pressure and more.

see here for some basic checks that you can do yourself with a multimeter. i would also suggest joining up as a passion ford gold member and then you have access to the technical essays archive that contains the complete yb engine manual. have a read of that, and you will understand how the engine management really works.

it's not ideal to be too rich off boost, but if that means that it is fuelled safely on boost then it's better than being spot on off boost but dangerously lean on boost. a professional tuner may be able to juggle the fuel pressure and poti to get it good throughout, and it's probably worth taking it to one to save fuel costs and oil change costs (that need to be done more regularly if it's overfuelling). being from peterborough, i think SCS are near to you and come recomended by a lot of people.