Since the conversion the rear cigarette lighter socket no longer worked so thought I would have a go at tracing the fault.
Took out the rear cubby and had a feel around but could not ascertain very much except that the wires to the socket were in place. So I stuck the camera phone into the gap and recorded a few seconds of footage (

)
On playback I noticed that the wire connected to the socket was white! Also the wires seemed securely connected to the socket.
So off I headed to the front of the van.
Now, contrary to what I was made to believe at the auto electricians, the white wires are not actually domestic three core wire! They are, in fact, a two wire cable (blue and brown). Wikipedia says that it is a 2-wire earthed DC power circuit wire. So not domestic after all.
Anyway, the two white cables have had their brown wires crimped together and the two blues crimped together with the blues going directly to the earth and the browns, via a bullet connector and a 25 amp mini fuse fly lead is connected to the positive. (Both on the leisure battery)
The crimping on the negative was rather pants and one of the blues had been sheared; the one leading to the rear cigarette lighter socket. A quick "bodge" later the negative is re-secured and the rear socket is working again. Yeah me <smirk>
So, my conclusion (for now) is that the two white wires coming off the leisure battery must be feeding each of the cigarette lighter sockets. I will confirm and label them as soon as I dig out the Dymo.
TTFN
Cuch