Finally finished with this after pottering for a couple of years. Made by a chap called Don Brooks, who made around 300 bikes in his career, and never really on a commercial basis.
Came to me via eBay, the original owner is now in New Zealand and has requested that I sell it to him if I ever decide to let it go.
The frame is a mixture of SLX and SL and the ride quality is amazing.
Running gear is all Dura Ace 7400-7410, including the stem (!)
Looking at the picture, it looks like the bars might need to be tilted down - but they're comfortable as they are.