I drive a 2006 Astra 1.8 SRI with 49,000 miles on the clock. A few months ago, the onboard computer indicated a 15-20 per cent drop in economy. I checked and found it was showing I had used more gallons than the tank holds. However, on checking the consumption on each tankful, I noted It was using 10-15 per cent more petrol compared with previous years and taking account of the longer journeys I had been doing. At its service I had a fuel line cleaned, which made no difference. Should I use some sort of engine cleaning additive or try a premium fuel for a few months to see whether that makes a difference?