Jam won't set if the pectin hasn't been released, to do this you have to reach the correct temp-see your thermometer as I can't remember what it should be. However if you use JAM sugar which has added pectin you don't need to cook it for so long and you get more jam/jelly for your money and a fresher flavour as it hasn't been cooked to death. You still need to test for setting as it varies a bit with different fruit despite what it says on the bag but as far as I'm concerned it never fails.

Thanks Sylvia

I used jam sugar and Certo and lemon juice and it just wouldn't set. I had it at a rolling boil for ages but nothing worked.

I have since read on another jam recipe that increasing the amount of fruit can cause a problem. The recipe said 2kg of redcurrants and I used 3kg. I proportionally increased the sugar but I wonder if the quantity was the cause of the problem.

I subsequently made a small quantity of redcurrant and rosemary jelly which caused me no problems whatsoever. I'm sticking to no more than 2kg of fruit in future.