I don't think it matters, but if you want to grow them in pots, compost from an ordinary bag will do.

I put the growbag compost in the pots as it gets them off to a good start but normal compost will do just as well, as you say Hywel.

makes no difference as long as you keep them regularly watered.

We do not think growbags are deep enough for a decent crop. We fill large pots, which we have taken the bottom out of, with compost and stand them on the open bags. That way you can get plenty of food and water to the plants.

If you have no large pots to cut but have some spare vinyl floor covering you can cut lengths , bend them round and staple them into bottomless pot shapes, and stand the resulting tube on the grow bag. Worked well for me. You can buy purpose made shallow trays to stand the growbags on to save mess on the nice new floor too.