Around 20 to 30 years old according to the chap who gave her to me, although this was vague. A chap at the local club can't remember sailing 'Braine' steered vessels as late as the sixties though.
Hull - strip plank on frame
Deck - One piece ply with 'drawn' on caulking.
Keel - Lead moulded and fixed.
Steering - 'Braine'
Length OA 40''
She sits in the water with a freeboard of about an inch and a quarter.
VMYG reply by Russell Potts
The boat appears to be a reduced version of Bill Daniels' Prospero of 1913. The original was 60 inches overall and rigged as a schooner. See link. The design was very popular, both in its original form and in a range of reduced sizes, many of which were rigged as sloops. See XPDNC drawing below. The design stayed in print in the little Percival Marshall 'Model Sailing Yachts' from 1913 till well after the war, so it's hard to be certain when your example was built. The use of a Bermuda rig suggests relatively late, say after 1930. The use of ply for the deck, if it is waterproof ply, puts the build date after 1945. The Braine gear was the norm from 1904 until about the mid 50s when seriously competitive model skippers switched to vane gear. But a 40 inch boat doesn't fit any of the recognised model classes, so she probably wasn't intended to race. So she could be as a late as 1960.
This is still not '20 to 30 years ago', but the design was around and could have been built to as recently as the 1980s, though I think it rather unlikely. Another possibility is that she is relatively early and originally had a gaff rig like XPDNC and was later converted to Bermuda rig. This seems rather likely as the style of the mast slide and Braine gear is much more 1920s than later.
If you are going to restore her, I would suggest that you do as little as possible, just making sure that she is watertight. If you wanted to put radio into her there are ways of doing this that don't do violence to the boat, though they need a bit of ingenuity.
It later turned out that the deck is probably not ply, putting the boat more firmly into the 1920s