I like this. I've long thought about building one, but every time I buy an inevitably cheap Q1 for the purpose I can't bring myself to chop it. I currently have eight of them, and counting. I think I'll end up building the 0-6-4T from plasticard, but I want to have a go at the 4-6-2 and 4-6-4T first. H.A.V Bulleid also mentions 0-6-2T, 2-6-2T and 2-6-4T variants, plus the bizarre 0-6-0 version with casing and two way controls. A 4-6-0 tender version of the 4-6-4T Q1 would look nice, or the 2-6-2 mentioned in the comments, and the two proposed dock tanks are neat (I built the second one a good few years ago, not a great model but it suits me). Is there a disease called Q1-itis?

I think the idea of using the back of a 4MT is excellent, and the mock up looks really good. I'm not so sure about the tanks. W5975 shows straight tanks, the top in line with the cab front windows, and the bottom in line with the bottom of the cab. Perhaps you could try trimming the tanks at the bottom rear, and extending them down at the bottom front with plasticard? Or mock it up with card to see what straight tanks would look like? That said, I rather liked the curved tanks, especially with the back of the cab on, and as the loco was never built there's no right or wrong.

That looks a lot better with the tanks raised.

John

Sometimes a prototype feature just doesn't look right on a model. The plan is quite clear about the tanks, but I think they look better the way you've done them, especially with the back of the 4MT. There's definitely a Brighton look. As I said, I like it. 

Glad to know the dock tank and the Pacific are on the works' order book. I'm going to start my Pacific soon, although I'm a bit of a butterfly (whichever project takes my fancy at any time is what I'll work on), so I expect you'll have a fleet of them before I finish mine. The Pacific and outside cylinder 4-6-4T will both have chain gear, because I'm sure Bulleid would have insisted. That this means far less complicated motion to model is purely coincidence.

I might put a photo of my dock tank on RMWeb, but to be honest I think it would receive a lot of criticism. I do like my thought above of a 4-6-0. I've never seen even a hint that Bulleid would have considered one. However, is it too much of a stretch to say that the operating authorities demanded some more S15s, and Bulleid thought we could do better than that?

I notice you have a Farish 94xx in another thread. This is getting spooky. Guess what I've been working on recently... I've found it to be a pretty satisfying model for its age. Some of the casting needed rubbing down, especially on the tanks, and the rear splashers are in the wrong place. However, sitting on an old Mainline pannier chassis that I had lying around, it looks the part - an impression rather than finescale. I thought about carefully filing off the moulded metal cab handrails, but life's too short.