Well Graham, seeing as how I have more time on my hands (these days) I did a little web-trawling and found some info on the brand. It seems K Yasuma built some very fine guitars in the 70s and depending on model have either plywood or solid wood tops (sides and back were laminated however)..... and they were of fit, quality and finish equal to Martin guitars of the time. Enough so that they got the pants sued off them in a lawsuit by CF Martin which drove them out of business.
Survivors are rare and in good shape, properly set-up, of a value that would put them up there with Yairi (another name from the law-suit)..... I have been finding three year old prices in the range of 500-1000 pounds depending on the top wood type. Generally comments are favorable from those who have owned or still own one. They "folded their tents" in 78-79 thereabouts. Could be a real gem!
ETA: The finish is nitrocellulose lacquer in case a little touch-up is needed and some guitar stand "rubber" can mar it from prolonged contact.
"what is this quintessence of dust?" - Shakespeare