Right, it's been ages and I completely forgot about this thread. So, here are some pics of the remainder of the project:
Floor stained:
Projector screen paintage:
You're damn right:
The business end of the cave:
My eldest playing site forewoman whilst I fit shingles:
First coat of stain:
Exterior complete:
Basically the cabin as it is now, there are a few minor changes that have taken place between then and now, but not much of real note:
So, what's it like? We've had temps as low as -6°C over the winter, and the coldest it's been in here is 2°C. An hour of the heater at full blast is enough to raise the temp to ~20°C. In those conditions, it loses about 2°C per hour with the heater off, it's really easy to keep it warm once it's there. I can't say how hot it gets in the summer, as I finished it right at the end of the last one we had. It's really, really cosy and basically feels like another room in the house. You don't feel like it's going to blow away in high winds; it's about as quiet inside as my house when it gets really blowy. Heavy rain and hail are quite loud, but not intrusively so.
There are no drafts, no leaks and it feels like it'll last for a very long time.
I still need to paint the base as it's a bit ugly and there's also guttering that needs doing, but it's in an awkward spot to route the water to a drain (for half the roof at least). I'll get around to it at some point. In the meantime, it gets heavily used and is my escape from the three women in the house!
I can thoroughly recommend Summer Garden Buildings as a supplier, we've had no issues with any of the kit on assembly or since. When we move, my wife already expects that I'll be building another one (albeit a bit bigger

).
If anyone has any questions, I'll be happy to help.
Before I forget (and someone will ask otherwise) - total cost was probably around the £7.5-8k mark to go from a bare patch of garden to what it is now (excluding furniture and electronics):
Cabin - £4700
Base - £1200
Electrics - £500
Insulation and waterproofing - £600
Wood treatment, stain and paint - £250 (I managed to get loads of tins of Sikkens Novatec & Novatop for less than half price as they were a bit dented - the cost here would have easily been double otherwise).
Other stuff I've forgotten - £a few hundred more