I have most of the 70s Model Boats in the loft, and have catalogued them, but in a fairly basic manner. I can find various mentions of the marine steam series but nothing about a model at the end (the cataloguing was done in something of a rush - I do plan to dig them all out and improve the catalogue at 'some time'.
Looking at the catalogue again, I can't find anything past part 15 of marine steam (that might be down to my rather rushed cataloguing) in February 1979. The following month featured a plan for the Lady Enchantress pleasure steamer, which might be the one you are looking for.
I need to get back in the loft and pull a few boxes of magazines down, so will prioritise the late 70s in order to check. Could take a while, though...