Thanks for your response and advice Jasper. Appreciated.
If I do sleep for those three nights up at 13,000 feet I'll definitely be taking it easy! The plan is to sit, read, take the occasional photo, and watch the clouds roll by! And with 4 days food on my back I won't exactly be rushing on the hike in, or out, either!
I'm still hoping that someone with a sound understanding of the science of how our bodies adjust to altitude will see this and respond. But I'll search elsewhere as well. And if I find a good answer I'll post it here.
The link you gave led me to someone I already know, a fellow Brit, who runs 'Active At Altitude'. I'll let you know what he has to say...