Try a proper coal man, you should be able to get 50kg for £17-19 we use supertherm bricks which are great. If you are outside the smokeless zone (or don't give a crap) you can get house coal but it will foul your chimney up a fair old bit. The stuff from the petrol stations like you used is not bad but it works out very dear.
I bought 1/2 a tonne in December, it's not cheaper than central heating but I love having a fire every day