Hi Richard
There is another photo of it in GWR London Division Engine Sheds (Hawkins and Reeve). From the pair of photos, it appears it was very similar to that technical drawing, although there are definite detail differences. There was an arm to move the turntable at each end (not just one end) and they are made of wood, not iron. There is a walkway of longitudinal planks at about railhead level between the railings and the track on one side only. (Possible these were later additions during service.) The cross timbers between the rails appear to be slightly raised (though not up to railhead level), unless it is flat bottom rail spiked to the cross timbers - photo is not clear enough to be certain. No details of the frame can be seen, and doesn't seem to have those levers on quadrant plates at each end either (not sure what they are, locking mechanism, maybe).
As for its length, I'm only going by what it says in the books, I don't know how precise the measurement was!
Hope this helps
Alan
PS If you are looking for photos of the Fairford turntable, it was changed for a 55' one about 1948 - there are rather more photos of this later turntable than the earlier one and I have seen one photo from 1961 mislabeled as the 45' turntable!