We put out a request for help to our friends and were very gratefully surprised to get four helpers showing up on Saturday morning (just) and displaying levels of enthusiasm that stunned us! We provided sandwiches for lunch, and curry and wine for dinner...
I had started on Friday evening by removing the staff beads and furniture so the first task was to remove the sashes. The inner came out easily, the parting bead was a doddle, but the outer sash took even more cajoling than I had expected; this was not helped by a very loose joint which we were trying to be careful not to stress too much. The sashes went out to the garden and the girls started stripping (the paint from the sashes, unfortunately).
The sealing around the outside of the frame proved to be very loose, so we decided to pull it all out and replace it. There turned out to be a large gap on the left side between the stonework and the frame, and on pulling out the cement we found that the gap had been stuffed with newspaper. Eventually we pulled out a fragment with the date at the top: Friday 22 February 1924!
Work went more slowly than I had expected (and there was more to do) but by the end of Saturday we had stripped all the woodwork, cleared out all the debris from the sash pockets, filled the gaps with expending foam, painted the cill with fibreglass resin and started the painting, so we all settled down to a well deserved round of Thai curry and New Zealand wine!
By Sunday morning, through our bleary, hungover eyes, it became obvious that we were never going get both coats of Ranch paint dry in time, so we elected to go and get a board to cover the window and take our sweet time over it. By the end of Sunday we had sanded down the foam and sealed the exterior of the frame with silicone, first coated everything and stripped all the paint off the brasswork, so we screwed the board in place over the window and sat back, pretty exhausted.
All we need to do now is second-coat all the woodwork and reassemble - most of the hard work is done. I think we've made more of a meal of it than perhaps more experienced DIYers would have, but then we've stripped it back to bare wood and started virtually from scratch. I'm so pleased with the state of all of the wood given its (minimum) age, and it seems worth doing a really good job and not cutting any corners.
Thanks everyone for all the advice. I'll post the final results when they're done, possibly on Wednesday (depends how much we can get done in the evenings).
Cheers - Adam...