I've just bought second hand a cannondale with a lefty jake fork.  

The suspension seems to be very soft and as there's no adjustment on the firmness, recoil only, what is likely to be the problem.

The guy I got it from said it had been sitting around in the garage for a pair of years and the suspension was a lot stiffer then. Judgeing by the cobwebs etc I believe him. 

Does it just need a service or something major?

It may just need a needle bearing adjustment, which is easy see here for Lefty servicing.

Not sure about the Jake but I'm assuming it would be the same internals as my old Lefty Max, you can pop the top off and adjust the spring tension also.