Having the same engine and worries in the past try this before you worry yourself unecc...let me know how you get on...
Reset your mpg computer whilst on a level dual carriageway or motorway or a road..
Travel for 1.5 miles at a constant 60 mph in 5th gear..
What mpg does the car compute ?
Should be around 40 to 44 mpg ?
If that's the case nothing to worry about..
Around town ( at the moment ) I am only getting 25mpg due to the cold conditions ( heater and auto choke - short journeys )..
, filter, plugs change may also improve things - use an expensive 10/40
to ensure better mpg and ths cost savings in the longer term..
Also can recomend an air filter upgrade ( k+n ) as this will improve your throttle response and not adversly effect the mpg or