Benji,
There are a lot of snake-oil fuel/air system cleaner off the shelf, but if you are prepared to chuck around £15.00 for a bottle of Forte fuel system cleaner, in my experience it's not wasted on engines used to a lot of short fuel-rich urban runs as opposed to nice motorway CAT-cleaning (etc) runs.
Basically wait until there's no more than 2 gallons left, shove it in, go for a short run, park it overnight, then the next day give it beans...serious beans....somewhere safe etc etc.
Forte stuff does what it says on the tin, but frankly I hold the view that modern engines, with 21st century fuels and oils, look after themselves between services.