different brands of petrol wont realistically make a difference.. maybe, going from the roughest of ghetto fuel, to the highest premium you can find, 2-3% better MPG presuming identical driving conditions.
Do you warm the car up idling for 5 mins, then drive 500m to work ? what sort of driving gives you ~16mpg ?
fuel pump could to be a good call ... double pressure tho ? WTF !
tenner at a time ? really ?! fill it up to the brim. properly full .. then wait til the red light comes on .. (by the sounds of it, this wont take more than about a days driving lol) then fill it to the brim again .. work out your actual MPG from a full tank.
If it's approaching single figures still, then get it to a trustworthy garage ASAP.