I am a complete novice and have heard that you can substitute grape juice from the supermarket for grape concentrate. If this is correct what amount of juice would be the equivalent of 250mls of concentrate?
Thats kind of a hard question to answer, because different concentrates are concentrated at different levels.
one might be 1.035 gravity, another might be 1.045 because its been concentrated more.
I will say that in the recipes section there is a frozen juice concentrate recipe posted. It uses 3.5 cans of frozen grape juice (i.e. Welches, Tree Top, etc) per gallon, and a bit of acid blend to balance the flavor.
Otherwise you'd have to get juice contentrate, and determine the gravity.