using lots of fuel??

i have a 1.6 gti,
 i put ?20 in the tank in bicester which took the needle up to 3/4 full, i then drove 50 miles at a normal speed to surrey and the needle showed just to the top of red part,
it seems to me i used a fair bit of petrol?? there don't seem to be any leaks (none i can smell anyway) and the car runs lovely
i assume the tanks are fairly small on these tintops but it just seemed to go down quite quickly  :dontknow:
please advise and hopefully reassure me

Fill it to the brim, do some driving (noting down how many miles) then fill it to the brim again, noting down the amount of fuel added. Can't really diagnose it, until you know accurately, what the fuel consumption actually is.