Check this website for different suppliers for E10. it will give you a list. I give you some names as well. Ring them up for a quotation. 27.5 pence is just too high. Some e10 suppliers are:
1- Our Power, contact no 08081893085
2- I supply energy: 03302020373
3- good energy: 08002540004
4- together energy
5- green energy Uk: 01920486156
There are few more as well but check these ones first. Hope that helps. Do update on the outcome.