[MatthewH]

Update: I have purposely not reset the engine module since the weekend I replaced the main fuel pump. I checked the figures again today after ~200 miles of driving and the long term fuel trims are now both around +7% and the lean codes haven't come back. MPG still hasn't improved though.

I recently remembered that at my last MOT there was an advisory for brake calipers sticking slightly (both fronts, one rear from what I remember). I was meaning to have a look but completely forgot, until last week that is. I was about to drive home and the electronic handbrake wouldn't disengage - the light was flashing and the motor didn't sound right. The offside rear caliper was seized and wouldn't free. I had to drive it home like this (stopping every couple of minutes so that brakes would cool down - the wheel was smoking and too hot to touch). When I got home I left it overnight and it seemed to work correctly again the next morning - the handbrake now disengages properly and it hasn't happened since.

I'm thinking this may turn out to be the cause of my MPG problems. I've never dealt with a sticking caliper before - would it be possible that I've been driving all of this time (sticking enough to cause 13mpg rather than 25mpg) without noticing it? It's never really felt like the brakes were dragging (other than the incident last week) but I'm not sure how much they'd have to be sticking to cause a big drop in MPG. It will roll down a hill in neutral when coming off the brakes.

I ordered a new set of discs and pads (front and rear) so I'll try to get them fitted this weekend and see if this improves anything.