I have a 1litre engine vaulxhall corsa..& its using petrol really quickly...every other I day or so I need to top up my tank,,,i average about 8 miles per day
So, how many miles per gallon does it do, 7 gallon tank only lasts 16 miles, I think there might just be something wrong if your 1 litre car only does around 2 miles per gallon!
a fuel leak sounds favourite here. the tank may have rusted, look under the car for drips: you can also smell the fumes on a hot day (typical british summer weather LOL)
or it may be a leak in the engine compartment, from the pipes that connect the tank to the fuel pump or the fuel pump to the carb. this sounds pretty dangerous, fuel leaking onto a hot engine!