Recently dicovered that the air pipes to and from the intercooler were really oily inside. Wondered if my turbo was knackered but didn't show any of the classic symptoms. Anyway, realised that there is a breather from the rocker box plumbed into the intercooler air pipes so that explains the oilyness.
Anyway, recently changed the cam belt and whilst the front end was in bits I decided to clean out the intercooler. Sure enough about 100mls of oil had collected in the bottom of it. Put a load of paraffin in it and gave it a good shake over the course of a few days then thoroughly dried it in the Rayburn (being careful not to blow up the kitchen!).
Anyway, now it is all back together but the big surprise is an increase in MPG from about 28 to over 31. Maybe doesn't sound a great deal, but it is an increase of over 10% and purely down to sloshing a bit of paraffin around in your intercooler.
Nothing else has changed so it must be the cleaner intercooler helping the whole turbo charging system to work more efficiently.
Hope you find that useful