Thank you for the advice. The jam was duly reprocessed and has set beautifully. Quite firm but it spreads easily and tastes wonderful. 
I emptied all the jars out, strained the contents, added 125ml of liquid pectin to the syrup and boiled it up again. I then put the pulp back in so as not to overcook the fruit.
I have never used pectin before. I would probably use powdered pectin next time as I think its cheaper and keeps better but liquid was all I could buy at 5:15pm locally.
I also tested the amount of pectin in the original syrup which was very low. (probably due to over ripe gooseberries.)
This is a fun bit of domestic science using a teaspoon of the fruit syrup and 3 teaspoons of meths. See below for details.
http://www.jamworld.co.uk/pec.html