Now what had stumped me was that I knew that the Collet Chuck fixed to the main spindle with four M12 bolts, and the socket caps for these bolts were covered by the collet closer front plate, itself retained by a ring of M8 socket cap bolts. I'd removed the closer, removed the collet, removed the four M12 cap heads, and could NOT budge the actual body of the collet holder.
Yesterday, I awoke to pouring rain, and decided that being a Bank Holiday I'd actually get up in a more leisurely way than normal. So there I lay, drinking my coffee and going through my list of things that needed sorting, and this conundrum came to mind. Then, in a flash of inspiration it came to me

I'd always relaxed the collet before removing it, and on this lathe the collet is PUSHED to close onto the collet closer front plate, not pulled like a normal collet. This meant that the mechanism was in this instance PULLED backwards,
as was the bit I was trying to remove (despite the power being off)
So today I had a couple of hours free as the sides of bacon I had to slice were chilling in the freezer. (Never try slicing bacon at room temperature, as it goes all over the place!)
So off to the workshop, CLOSED the collet, powered down, gingerly removed the closer plate that now was being pushed off), removed the collet and was able to unscrew the collet closer from the operating tube and remove the body of the collet