By Ilaria Grasso Macola Drivers should have seen a bigger cut in fuel prices at the pump after costs slumped in September. According to data published today by the RAC, the average price of a litre of petrol went down to 162.9p – the sixth biggest monthly drop since 2000. Filling up a 55-litre petrol car has gone back to below the £90 mark for the first time since May, while the cost for diesel went down by £10 to below £100. But savings for motorists should have been 10p higher if retailers had not hiked their profit margins. “Despite September seeing the sixth biggest ever drop in the price …