Thanks for the replies!
Car still the same, average mpg sitting around 50 now, after swapping MAF sensor to a known good one.
Something else has started to happen though - I now get what I can only describe as a misfire/loss of power (usually at low rev, but not always).... Usually only lasts for a second or so, but it is getting worse! No management light coming on whatsoever. Only code the car is throwing out now are; 0048A7 (unknown failure), 004089 (Unknown DTC fault), 003EED (Unknown failure/Power Management/Closed current Power Management)
Starting to think this is a fuel feed problem....? Would the fuel pump cause a loss in MPG??
(The start-stop feature is working!)