The other person is in my company, indeed, and I will take the CHM home this evening. In the meantime I have scoured the output view for anything approaching an error. There were a few pages which allegedly had illegal characters, I have sorted those. And there were several entries along the lines of:
HHC4014: Warning:
The following map definition has an invalid context ID: #define (No assigned Map IDs) 0.
A couple of them were just like that, another couple had names and numbers. I have resolved all of them. There is nothing in the output view now that looks at all untoward.
Interestingly, when the help freezes having been viewed in RH, closing the help from the task bar brings up
Does that shed any further light?