Carrots in containers

I have always struggled with carrots and carrot fly.  Even Flyaway got chewed.  but they are lovely fresh in salads so I am planning to try with them in containers, to keep them above flight lines.  I read that the mix should be 1/4 sharp sand and 3/4 compost.   So two questions:
1) not wanting it to be too rich, should I use growbag compost (not in the bag, obviously!  )?
2) what the the best deal around at the moment?  There is a homeb*se, beenandqueued, wickes all in range (different directions of course) and an independent.

Try and blag some 4" drain pipes about 4 feet long and fill them with your mixture. As long as there are no lumps or stones you will get exhibition quality carrots (and parsnips)

Thank you all!