As a follow up to this (and in case anyone is vaguely interested) I contacted J Costa directly and they emailed me an installation manual, different to and much better than the one supplied in the kit. However, the new instructions verified I was installing everything correctly - it is a very basic installation anyway.
I also took their advice and ordered and installed heavier 14g weights.
** Even with heavier weights, the J Costa variator still DOES NOT WORK **
OEM variator = 105kph (65mph) at 7000rpm
J Costa variator = 70kph (43mph) at 7000rpm
Both J Costa and my scooter shop have now gone quiet on me.
I can only assume that there must be an incorrect part in the kit, but J Costa is not forthcoming at all about any information. Very frustrating
In the meantime I have ordered some Dr Pulley weights to get me back on the road again......