Thanks for the estimate man, appreciate it.
At the moment I drive from the coast to Norwich, which is roughly 25 miles (3/4 motorways, 1/4 town/city), so there and back is 50 miles. I top up £25 a time and a trip to Norwich usually drains my car to 3/4. So that means i'm getting around 200 miles per £25.
£12.50 per 100 miles, which i'm guessing is about right considering i'm a first timer.