£32 buys about 24 litres, 300 divided by 24 = 12.5 miles per litre, multiply by 4.55 to convert to mpg, that's 57 - you're doing pretty good if I've worked that out correctly.
My feeling, with my bikes, is that speed drinks fuel, especially upper motorway speeds.
does despatch riding pay? Once I lived in a group of flats (converted house) in East Dulwich, SE London - there was a despatch guy in the attic rooms, wow his place was a mess (lovely bloke) - just worked all hours, this was back in the 1980's.