This year I have used the John Innis seed compost for sowing and that has worked well for me. Local GC was around £3 a bag. I also bought some New Horizon and I can hand on heart say this is the worst compost I have used, the seedlings went yellow within a couple of days.
I have gone back to one I have used before with good results... Jack's Magic which is a reduced peat compost. Plants healthy an d strong.