Just a quick update. After messing about with additional headshell weight I settled on 5g so with the Jelco headshell at 12g I'm running around 17g total. So Marco's advice spot-on. I also tried some loading plugs and settled on 150pF ( which obviously might vary in other set-ups but sounds great in mine).
Now I have to admit that initially I was feeling underwhelmed by the sound produced and started researching new carts. However, I'm not one for making snap purchases and it dawned on me some weeks ago that I was now enjoying music very much. I think the NOS stylus just needed running in for a while. I now hear balanced sound ( what I mean more specifically is a natural frequency range, no HF emphasis which I detest) , good detail, separation of instruments and very low surface noise ( in fact pretty much non-existent on good vinyl).