Well Adam I'd suggest either once a fortnight take your car for a 50 mile run with the occasional 'Italian tune'. To me you are strangling your car and probably coking it up. I also suggest you use E5, if you're not already, and I find adding a dose of Millers petrol ecomax helps a lot. I've been using it in all my cars for decades.
My journeys are short like yours but I take my is250 for a blast once a week to clear the cobwebs and its fun also lol.