OK - so anyone reading this might be interested to know I have 'fixed' it by doing the following:
Home > Help & Settings > Settings > Video Output > Video Output Format > Test Formats
Then went through the test and 'thumbed up' all the video formats that I could see during the test. All formats work except '2160p 24/25/30fps (pass-through)'
Note: Since install (5 days ago) the v6 box was on auto-detect and 4k output for iplayer and youtube worked perfectly fine until yesterday. My TV is also a 4K/HDR TV so was surprised I would get a problem.
OK - so latest on this is that I had to switch to Auto Detect as selecting all outputs, including 2160p 24/25/30fps (pass-through), I saw the same issue as before in that iplayer and youtube were playing up with sound but no image.
Essentially the video output is a little flaky.....the question therefore still remains, has anyone experienced this with iplayer and youtube?