OK, a little more info (took a while) on the hydraulic drive. Here's it cleaned up a bit :
It is indeed a variable-speed drive, where the input shaft runs a hydraulic pump which in turn drives a hydraulic output motor. The swash plate is on the pump (input-side) and can be angled using the control wheel (which also has the 'speed' dial set into it, with the red needle) that turns through about six complete turns, three either side of the zero position. In 'zero' the swash plate is flat, no output pressure is generated and so the drive is effectively in neutral. From that position you can set the output speed to be anything from about 4% of input, up to 100% of input, in either direction, with (allegedly) full transfer of torque. I suspect there will be some energy loss from all those mechanicals, but that probably explains the sheer size of the motor (see earlier in this thread). The makers reckon about 85% efficiency.
Interesting stuff. Renolds (www.renold.com) were indeed very helpful at getting background and even service manuals on the drive - thanks for the pointer AndyT - they were a bit intrigued at the age of the device, from its serial number it was made in 1969 but they are apparently "good solid units" made between 1953 and 1980. If it does what it says on the tin, it's pretty interesting option for running the lathe, so I'm going to continue cleaning up the motor and see how we do. Watch this space.
While I'm here, another question : what sort of paint do people like to put on machinery they're fixing up ? I have used Paragon Paints enamel in the past, which seems good quality and comes up to a very nice gloss, but I'm wondering if that isn't too shiny for some workshop machinery ? Lathes seem to want a slightly flatter, more 'working' finish, especially for the bed which is quite a large area. It must be possible to get good strong-but-not-high-gloss machinery paint but it's hard to know from looking at a website. Thoughts ?