A visit to the British Library showed that the turntable was indeed near the engine shed in the 1870s, according to a six-inches-to-the-mile map (which I forgot to record and will have to revisit to record the source and date).
I assume, therefore, that the three sources all use this old map as the original.
I have, however, a video of Beattock station in the 1950s & 60s (Great Steam Routes: The West Coast Main Line), which shows (fleetingly) the position of the turntable. Another video (North from Carlisle) has better footage of the turntable itself but the location is less discernable.
So it's possible to guesstimate where the turntable was.
I've used XtrkCAD to plot the track plan, allowing for some simplification and compression (otherwise it's just too big and unwieldy) - operationally it should be fine.
The overall track plan (omitting some sidings and structures) looks like this:
The baseboard will be made up of four 4 x2 boards, with two 2x2 boards at either end as turning circles. I had hoped to squeeze it all on to four boards, but the boards then become too wide, or too short to create the visual effect I'm after.
It's basically a glorified dumbell layout - allowing long trains to run through uninterupted if desired, but goods shunting, banking and a branch line to maintain interest.
The station throat, goods yard and engine shed complex in detail looks like this:
There are plenty of points to install, and I'll need to check whether these coincide with baseboard joins or bracing.