If you start Thunderbird in safe mode is it better? You do this from the help menu.

Thank you Airmail. When in "safe" mode the system works as it should when replying to a message. It would appear that one or more of the add-ons is causing the problem - just need to find out which one ! Thanks for your response.

Hmmm ! Have disabled add-ons one by one and have also disable all add-ons - restarted program each time when prompted and I still have the same result. It is only in "safe" mode that the program appears to work correctly. Is anyone else having this problem? Not sure what I can do now to resolve.

Thank you sfhowes - I ahve tried all ways as you suggest and now understand the process. However the bug is still there but I can live with it. Perhaps it will be sorted automatically when next updated.

Do you have add-on Display Contact Photo installed? This add-on is known to cause this problem…

Thank you Onner Ekker. I had previously disabled all add-ons and still had the problem. I have now just disabled the "Display Contact Photo" add-on by itself and hey presto the problem is solved ! Thanks very much, your help is appreciated. The "DCP" add-on is really just a gimmick and I can certainly do without it ! Once again many thanks.