1*2s have 11gallon tanks. Nearly 50litres. I think non-sports are the same. I regularly fill up 45+ litres with the red light on.
£30 is roughly 25 litres, 100 miles = 4 miles a litre, roughly 18mpg. That's way below normal even for winter driving. I cant get that low even driving lead footed in my 172.
In my old mk2 ph1 1.4. I consistently got 30mpg for 3 years but I mainly drove it when I came home from uni so wasn't light footed, then for one summer I got it into the 40s during brisk commuting (mine didn't have a trip so no false numbers).
Brim it up, run it to the red light and brim it up again. See how many litres you put in the second time and how many miles you've done between filling up and that'll give you as accurate an mpg as possible.