38.26 litres? are you sure? I thought its a 35 litre tank.

will report on my tank this morning as I need to refill.

That's what everyone thinks, but if you really try, you can get 42+ litres in! My most was nearly 40 
That's why i tell people not to panic when they see the warning light come on, as there is a lot more left than what people think. I usually fill up 10 to 20 miles after the last tiny light has gone out, and the range has been reading ---- for around 40 miles!

I'm a tad  , more so at how much my driving style looks to be shaping up to most of you

I'm going to blame the lack of motorway driving/constant conservative speed, down to the fact that I just don't see my MPG start with a 3 after a full tanks gone.

You must have been doing it wrong, the computer is consistently around 2 to 3 MPG higher than actual. The way to work out how many litres you've used since the last time is to brim the tank every time (until you see fuel in the nozzle end) It's the only way to get consistent/accurate data.

Basically, the light comes on when there is a quarter left, which soon turns into the last 2 notches. Shortly afterwards it beeps and says "warning low fuel" and it will do this every time you start the car. Then it will go down to one tiny light and eventually that will go out too. But there's still plenty left, so i really wouldn't worry too much. One day, i'm going to take a jerry can of petrol and wait until i run out, i really want to solve the mystery of how big the Fuel tank is! 

That's not the way to do it. Lets assume you start with a brimmed tank, you reset "trip A or trip B" so you can see how many miles you do. Then when it's time to fuel up, whether it's half a tank or nearly empty, you brim the tank again, this way you know exactly how many litres you've used and how many miles you've done according to the computer. So take a note of the litres you put in. Then fuelly asks for the miles done and the litres used and it works out the MPG from that. Get it now? 

No, it doesn't matter how big the tank is, or how much you put in! Could be 42 litres, could be 2 litres, it doesn't matter, you're going by the distance travelled divided by the litres used. So if i filled up at half a tank and i got 15 litres in and did 120 miles, then when fuelly asks me, i put exactly that in, miles travelled and litres used. Get it now?