i was going to do a post to see if anyone has tried this but i spotted this thread and it seemed appropriate.
product forte injector cleaner, also an aerosol cleaner that takes about a half hr to do, you have to keep the engine running at 2000rpm and spray the can into the airfilter until its finished - then you need to change the oil and filters a garage will charge £100 i think but the mechanic in the family says it makes a massive difference, and the garage where he works do a lot of taxis and they all say the difference is noticable
to buy the injector cleaner and air aerosol cleaner they are £10 each.
my millage was around 320 to a full tank but ive got the van with tools nobbly big tyres and just had the exhaust done which has made a massive power difference i kept the old cat as that was fine .