[grinling]

I don't mind reboiling it, should I buy pectin sugar?

[Snowpea]

What proportions of fruit and sugar did you use?
According to my preserving bible redcurrants are high in pectin.
It recommends 0.5kg (1 lb) sugar per 575 ml (1 pt) of juice. Note: don't mix the imperial and metric measurements.
You may not have boiled it long enough, if so re boiling may help.
Depending on how much sugar you have already added I would be wary of adding more.
My fix for non setting jam/jelly is to buy some commercial pectin and follow the instructions for poorly setting jam/jelly. Works every time. Not sure if I'm allowed to mention brand names here and besides I'm in Australia so I'm not sure if you would be able to get it in the UK. 
As for using pectin sugar, I never use it. I just use standard sugar. I have a list of fruits and their pectin and acid content and follow the rules for when to add pectin and acid according to how much is in the fruit.
The book I use for all my preserving ( not just jam/jelly making) is
Sonia Allison's Book of Preserving published by David & Charles 1979   
It may be old but I've never come across a better book and it hasn't let me down in all the years I have had it.
Hope this has been of some help.

 

[grinling]

That is how much sugar I used. Will track down pectin.