I just did the trimming in a new browser (Windows 10 Edge) and although the error showed up while saving, the recording problems with BBC 1 Wales SD has been fixed. I set a recording on BBC 1 Wales SD and it showed the correct times in the reservation window.
So my conclusion is that Google Chrome is causing some sort of problem when trying to save using Webif, and I shall avoid using it in the future.
I am also quite convinced that I have had this same problem, about missing recordings in the past year or so. I have had no explanation other than to assume we haven't set it correctly or forgot or some other excuse. But I am now convinced it was this problem, and I never noticed that the reservation was wrong. I will continue to monitor until I gain confidence that the problem is truly solved, and report back if it isn't.
Thanks so much for the support, which got the little grey cells going !
