I'll be honest and tell you i'm in Aberdeenshire.
-That said, i agree with the guys above.
Secondly, if you've got contaminated fuel - it won't just be your injectors, How about your filter, pump, lines - in fact everything from the tank to injectors.
This is something i'd attempt myself - if i was that stupid -but you're talking at least a day if not more - and that's with a limited tool kit.
Take it on the chin and go to a garage - it'll save you a lot of heart ache and put down to experience.
As far as HMRC are concerned, they turn up in the carpark at the bottom of my track regularly dipping tanks. Last Farmer who got caught, Lost his Landy and fined thousands (in lost revenue - story goes that they worked out how long he had landy, how much fuel he would have used and worked out the tax he should have paid) - Does that sound right guys?