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.