i personally prefer b&q peat free compost(150 litres for £6 or less),some perlite and a little gravel,personally i use about 5 parts compost,2 parts perlite and a half to 1 part gravel,helps get the water through all the medium and makes overwatering (in my opinion 1 of the worse mistakes people make,overwatering) harder to do
but that's just what i use,through trial and error,i'm sure plenty other 'better' mixes will appear here,it's going to be whatever suits you best