Many thanks both for informative answers. I kept looking at it thinking “why is that way up?”, but because that was how it was I did not like to change it over. Based on another quick fiddle with it last night, I would say the track on the mast is now fitted as high as possible, so the sail is set with the slider at the very bottom. I have not really peered that closely at the top to see if there is space left on the slot. If not I might move the track down an inch or so to get a little be more flexibility in the position. I do quite like the general head room from the high boom, not being a racer. This would also mean the boom will sit more easily on the transom when putting on the over-boom cover.
I can see how helpful it would be for reefing, which I had always assumed would be done in the same way as you might do on larger craft. The rest of course, (especially the beer), makes perfect sense.