I was not fortunate enough to see Sandy Denny in her early years (1960s) but she favoured a Gibson acoustic as well (round shouldered Country Western from the late 50s) and got a lovely sound. She bought it from a visiting American - along with other US-made instruments (arguably the best back then) they were not commonplace in the UK shops at the time I believe - which led for a while to the emergence of some UK brands to fill the gap; most of these (apart from the boutique makers) disappeared in the early 70s as the cheaper instruments started appearing from Japan (many of the latter initially straight copies of Martins at half the price, or less). When I was starting out (early 70s) "cheap" Yamahas were the order of the day for the starting-out player; in my mind they are/were mostly associated with lower-end instruments but they did make some, little advertised, high grade ones for e.g. Bert Jansch as stated (but those never appeared in shops, at least not the ones that I frequented!).