What's a suitable turntable to use? The obvious choice is the Peco NB55 model, and according to the catalogue is "based on a Ransome and Rapier 73ft well type turntable, supplied to many railways both at home and overseas." In other words, it's generic.
The turntable deck is 151mm (6.1 inches) diameter. It's just too big to fit comfortably on the layout, according to XtrkCad. The turntable at Beattock was "a balanced 54ft unit from Cowans Sheldon nearby at Carlisle" (source: LMS Engine Sheds, Vol 5). So it was 19ft (36mm in N gauge) smaller than the Peco effort. It was probably only used to turn banking engines - a Duchess or similar would never have fitted on it.
Re pictures, there aren't any. The only source I have is my North from Carlisle video (B&R videos, vol 33) which has a scene where they turn CR439 class 55234 prior to towing it off to be scrapped. You can see that it's a well-type table, but barely long enough to squeeze the tank onto.
My plan, such as it is, is to buy a secondhand Peco version on eBay and cut it down. I suspect I'll need a few goes at this, hence using the cheap option.
***Update: the video source is Cinerail's North From Carlisle, not B&R.