Well, Glenn has since been in touch and told me about this British Stamp Exhibitions catalogue he had co-produced, which would include all the information I was looking for. So I got myself a copy and started hunting for more items!
Turns out that there were three sheetlets of this kind issued, in 1973 (Seahorse), 1975 (Empire Exhibition) and 1979 (PUC congress), all using engravings by JAC Harrison. All three were produced in two colours: a regular colour, which was made available at the exhibitions, and a second colour for presentation copies. The latter of course being the more unusual because they were printed in limited quantities.
I've been rather successful (again with the aid of Glenn) in tracking them all down, so I'm able to show you them all here in this post.