Thanks for all the help. It was really reassuring to know that all that was needed was a bit more brute force in the right direction.   Got the mirror off now, and, of course, once you see how it is held on it isn't a problem for the future.  I will certainly connect a strap to hold the mirror if it comes of inadvertently, like hitting something or having a car door open against it etc.  Thanks again.  Now for the challenge of batttery removal  (sigh).