Someone been syphoning your petrol?
You can't use the gauge as a reference, its not linear. The top 1/3 or stop drops really quick then it hangs around in the middle for ages.
cold weather will affect your mpgs a bit...
I reset my MPG @ my last fill up & although my trips are short I'm only seeing 22MPG @ the mo
Nah i reckon you will be fine mate. If you dont "brim" it a lot you wont have noticed this. but for some reason (dont ask me why lol) i find that the 1st 1/3 of a tank goes in like 60 miles, then the next 1/3 last an eternity and then the last 1/3 is sort of normal, its as if they made a trapeze fuel tank lol
mines killing me atm, 27mpg used to be 32ish