Ford Focus Fuel consumption?

I have a 2012 (62) 1.6 Ti-VCT (petrol) Ford Focus and the trip computer is saying that I'm only getting 25 miles per gallon around the town etc (urban) The owner's manual says 35mpg! The car has had a full service including spark plugs.

Also when considering how much petrol is left in the tank would you use the trip computer or the pointer on the fuel gauge to judge how much petrol is left?

The fuel economy rating is taken from a highway situation of 50mph (80kph) constant speed on a smooth flat road, warm weather, no accessories running like A/C, fans etc. So real world you aren't going to reach that, especially in stop/start traffic, A/C on, really cold weather

The published fuel economy is from a specific test. All cars do the same test, so it's a useful comparison between different models. But it bears NO relationship to your real life economy.

Reason is that in real life you spend some random period of time waiting in traffic or at stop lights, effectively getting ZERO mpg at that time. This drags your average mileage down horribly.

Now you might be able to get 35 mpg carefully driving around a flat course at 30 mph, with an occasional stop sign. I'm not sure on the exact test parameters, but it's something like that. Back in the real world, more stop starts and idling, and you fuel economy drops. How much is hard to predict, because it depends where (and when ) you drive.

I'd fill the tank, drive till empty and refill the tank again. Divide miles driven by gallons to fill your tank. There's your miles per gallon.
The EPA mileage is almost never accurate, especially for city driving.