To do it for �50 (assuming fuel at �1.09 per litre) your car will have to achieve 55 miles to the gallon. This is unlikely unless it is a diesel, then the fuel will be dearer.
Do you know how many miles to the gallon your car does?
If so, divide this number by 4.5 (the number of litres in a gallon). This will tell you how many miles to the litre your car does.
Then divide 550 by your answer from above (this will tell you how many litres you need). Then multiply the answer by the cost of fuel per litre.
Example (if your car does 30 miles to the gallon and fuel is �1.09 per litre):
30 divided by 4.5 = 6.7. (Your car does 6.7 miles to the litre)
550 divided by 6.7 = 82 (you need 82 litres of fuel for 550 miles)
82 times �1.09 = �89.38 (the cost of the fuel)