It's a shame that there are no accuarate RTR milk tanks avaliable but there are perhaps reasons for this byond manufacturers simply getting things wrong. There are so many different variations to contend with particularly with regard to GWR and WR ones. The LMS and SR were more consistent but even still there were differences. Swindon issued 24 diagrams for 6 wheel milk tanks and they did this for a reason, they were all different! On top of this there were the Ro-Rail tank trailers and early 4 wheeled tanks that were converted to six wheel. There were two very different types of brakegear, twin tanks, twin compartment tanks, some that were a foot longer than others, different ladder arrangements, a myriad of tanks supports, platforms at the end, platforms in the middle, lots of different valve arrangements on top of the tank.... Which one do you go for? Most of the diagrams were dairy specific so it's not a case of pick one and you can paint it a load of different colours, well not if you want to be accurate. The LMS ones are the more obvious candidates which is perhaps why Dapol have gone for this. I have a long standing ambition to have a model of the Whitland to Kensington as running in the mid 60s and I think I could have legitimately have 20 milk tanks belonging to the various dairies serving South Wales and all of them could be different!
Whilst there are no accurate 4mm RTR models David Geen makes a quite lovely kit for the LMS Dia.1994 and GWR Dean-Churchward braked types. This is one of the GWR type I've finished to Dia. O.42 in P4 with some mods along the way:
There are some things that you might want to have a look at Dave:
As Paul Bartlett has already said the underframe is an issue as it's so open but you already know about that and I saw that it was different on the original CADs you posted on another thread.
Brake levers. I'm not sure the LMS ones used lifting links at all. They had simple levers on both sides acting off the same cross shaft which was closer to the outer axle.
Tank supports: The larger of the two types looks to slope inwards too much. Most of the larger type that I've seen on LMS tanks have the sides more vertical and are more rectangular in overall shape. More like on my David Geen LMS kit which also illustrates the point made earlier by Paul Bartlett about the underframe and also shows the brake cross shaft:
Spring hanger brackets: The LMS ones had longer brackets on the centre axle. See here for a good illustration. No idea why.
Hope that's of some help.