So, 6 - 7 kg plus water - I was not far off in my estimate. I do regularly run my long runs carrying about 4 - 4.5 kg including water, but it looks like I'd be at least doubling that.
I've been looking at prices of lightweight tents and stuff for a while. Lightweight tens appear to be £300-£400+. Sleeping bags £150-£200+... Gets expensive to do one of these events! Not so bad if you're backpacking regularly and gradually buying better gear, I suppose, but a lot of outlay otherwise.
As far as navigation goes, I've done a bit, but there's a reason I'm going on the Sunday navigation course after the November Lakeland 50 recce... And I don't think I know anyone else who would be interested.
Oh well, something to consider for the future, if I do find a potential OMM-ing partner!