It's probably no consolation whatsoever that both the Ruston and the Manning Wardle would have been my choice of prototype in your position John. I do have fairly weird tastes, so perhaps asking my view and doing the opposite would give a better chance of commercial success.
I would say, speaking personally and also following conversations with friends, that a Manning, Wardle 'K' class would have done better than an 'L', not massively so, but better: there are those who would have bought a kit for either class, but more who would want a K rather than an L specifically.
I do know and understand John's reasons for doing the L, though, but will not reveal a private conversation in a public forum.
I'm surprised at the relative number of western region hydraulics in 1/32 that seem to have been bought and built, but have they been general sales or targeted at a small group of like minded souls who may have instigated their production? I do wonder if the 1/32 market is too small (and too fragmented in its tastes) for the purely speculative kit rather than one that may have the backing of a number of pre-orders?
Some history for you, Neil.
Bob Hunter is a great fan of NBL diesels - especially hydraulics - and essentially put a lot up front for the production of some kits.
Something like a Warship will sell well, as it can be stuff full of batteries and r/c and be running quickly round a garden track if visitors call. A 47 would have sold just as well, maybe even better.
The 22 goes with it nicely and is a popular prototype: it has not sold as well as the Warship, however, but it has done well.
Sales of the 21/29 have been single figures.
Thing is, in 1:32 scale, these beasts are between 18" and 22"+ long, and with something as sharp as a B6 turnout taking up 2', there isn't much you can indoors beyond a loco depot, so building a number of these kits is a logical thing to do, especially as there are not (yet) many wagons to be had, and even a humble 16 ton rot-box is about 8" over the buffers. Also, of course, hydraulics were concentrated on one region, so if you have one, you probably want more than one...
Personally, I would have much preferred a Derby type 2, but I think what we had here was the happy coincidence of the right kit coming along at the right price, with the right market in place at the same time.
Don't forget that these kits - which are still state of the art, and still streets ahead of most manufacturers - have been around since the late 1990s, so there has been no great rush of them coming into service, but after people producing a finished loco every year or two, there are suddenly quite a number about.