Thank you for your suggestions.
1. I am not sure that I understand what you mean by “the level of the source recording”. Please advise.
2. I tried the avc-intra route and this also produced an unplayable file. I assume there is experience of rendering to ProRes UHD formats and perhaps the problem lies with my source material. This was acquired with a JVC GY HM170 at the higher record rate of 150 mpbs and is a .mov file.
Thanks
Ned Cordery