Hello Hywel, I have a copy of The Dinghy Yearbook for 1958. There is a fascinating piece in it by Ian Proctor entitled 'Dinghy Hull Shape'. He talks a lot about the developments in the Merlin Rocket class.
The relevant part for you starts off talking about his MkV1 design, which was his most successful at the time, then I will quote you what he says: "In fact in 1955 we seemed to overlap, for Jack Holt had a very flat and powerful boat, which was excellent at marginal planing speeds, and I had an exceptionally heavily rocketed boat - the Mark V11 - with very vee'd bow sections but a flattish transom. I also designed a Mark V111 that year, with a more heavily curved rocker, rounded sections and a very low wetted surface, but which at the same time retained the planing features of the earlier boats; she was intended to be a fast river and estuary boat, where quick spinning from tack to tack is important, quick acceleration, good ghosting ability, but also good planing ability. She proved rather hard work for a lighter crew to hold up in a blow and was slowed up in a chop to windward unless she had a heavy crew to give her the weight to push her through the waves. She did , however, successfully do the other things asked of her and some of these Mark V111's have won prizes both inland and at sea."
So this bears out pretty much what the others on this thread have said. When Ian Proctor was writing everyone was striving towards a fast sea and all round boat (just like they still are) but now you, or others like you, won't be looking at buying this boat with an idea of winning the Championship but of winning club races and Vintage series events on restricted waters, so this sort of boat could be ideal.