That's about the same as my dads car.
I think mine did 30-40ish. Hang on:
50l = ~11 gal
brum - aberdeen = (~430 miles, which pretty much rinsed my tank)
430/11 = ~39

In practise that was at 70 the whole way, driving at around 90 cost me an extra tenner. which would be around 2 gals extra, giving around 33.
( Slippery Mick ‏, Mon 8 Jun 2009, 14:56, archived)