@James another update Missing HDRI resolved - all OK. I can select GPU & CPU, what's the best option just GPU or both? I had to do a couple of reconnects to get it running but it looked good for dropouts. Performance needs closer monitoring - time-per-frame stay the same even though both nodes appear to be rendering. I'll try with the system monitor. I tried using Windows Remote Desktop to check what was going on but that usually crashes the remote Blender (no suitable graphics). It's 3 floors away so it would be useful, exercise is good I suppose.

Phew, quite an answer, thanks very much James. To your points. Colour differences: Embarrassing but true, I am using an external HDRI and I didn't pack it into the blend file - I will correct that. Speed: Actually I did try to change the crowd-render settings on the local & remote machine to GPU (CUDA), but I got the impression that it sometimes got reset. I'll try again. Dropouts: I have a 1Gb wired network connection for the 2 machines - no WiFi. I'll run a few response time checks. I think I noticed a comment somewhere that you don't like IPv6? I keep switching it off but after OS/driver updates it sometimes gets reactivated. Could that have an impact? In the meantime I relasie I can just r-render the missing frames, there aren't many I'll keep investigating. John