By Guy Taylor Supermarket retailers in the UK are still jacking up petrol prices at the pumps despitenearly a year passing since the government pledged to intervene, the Energy Secretary has been warned. In a letter to Claire Coutinho, the motoring group RAC argued if retailers were fairer on British drivers, diesel and petrol should be on sale for around 145p, compared with current prices of 157p and 150p respectively. According to the RAC’s analysis, diesel margins have been above 15p per litre since April 22 and increased to above 18p last week. The margin on petrol is now nearly 12p a litr…