Did you change the Air Flow Meter at the same time as Air Filter in December? These affect performance dramatically and if a cheap aftermarket jobby might just be culprit.
 
A bit obvious, but not a leak anywhere is it? Petrol with evaporate without leaving a puddle anywhere, and in this chilly weather will not be obviously smelly unless garaged. Tyre pressures & binding brakes already mentioned, you`ve not fitted a huge roof rack & top-box, have you!
 
no roof rack, tyre pressures all OK, dont think theres a leak. The mpg display would not be affected by a leak would it - thought it was a measure of fuel injected into engine??
 
I saw O2 sensors on a few posts back ....

Kinda jumps off the page for me - A duff lambda sensor will cause the system to default to ultra rich ....... had it on my 4.6 .....

Stealer charged me 80 quid to plug in and confirm the fault. 15 min fix .... sorted.
 
Re last answer: pull out a coupla plugs and check their colour. Brown isolators good, Black, sooty = bad.
 
That *.txt file shows no reading from Bank A HO2 sensor. It could just be the hardware you used.

Not being one to state the bleedin' obvious, but wouldn't it be worth the cost of a 10 minute T4 session (probably £25 worth) to find out what the stored DTC's are?
 

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