I should perhaps have added that although the owner was thinking about Merlin as the builder, this was only in the sense of rarity value. There is no question that TST, then a subsidiary of Sandvic, is a very high quality builder and up there with Merlin in the as-good-as-it-gets category. I can only guess, but it wouldn't surprise me if Merlin never built a Hei Hei, in which case this would be one of the very first ones made.
It looks to me like a size 18 (or what they called an 18 in 1991, with an 18.75" seat tube), but I recall that the serial number says it is a size 19. That could mean that TST built to a slightly different geometry at that point, as they certainly did from 1994 to 96. Not a problem, but worth establishing that the top tube length is to your liking before you make an offer.
If the size is right, I would guess this is a very good bike. The caramba kid has two early Hei Heis if you would like an expert's view.