45.84 pounds
OK. This problem isn't that hard if you just take it in pieces.
First, calculate how much oil Mr Leonard needs to buy. That will be the capacity of his oil tank minus how much oil is already in it. So
1200 litres - 450 litres = 750 litres
Now figure out how much that 750 litres of oil will cost at full price.
750 litres * 81.5 p/litre = 61125 p
Now, calculate how much is 7.5% of 61125 p
7.5% * 61125 p = 0.075 * 61125 p = 4584.375 p
Finally, since there's 100 pence in a pound, divide the number of pence saved by 100. So
4584.375 p / 100 = 45.84 pounds