Thank you for the replies Dave & Andrew.
I've just been around to the lock-up where the boom is kept, with the intention of measuring the fittings, and forgot the tape measure!
There is a slider track on the underside of the boom, roughly in the middle and with two sliders, both used by main sheet blocks. Before I posted here I assumed the kicker shared the rear slider, which is at the end of the track.
I think the next owner will want to update a lot of the rigging but I would like to get it into a usable condition before I move it on.
I'll do an update for Youtube soon to make it clearer.
In the meantime I'll check out the CVRDA.
Thanks again, things are a bit clearer now.
Floyd.