Thank you Brink both for your welcome and reply. You are right,

I didn't go done far enough, perhaps the result of trying to work on when too tired to be doing so. Especially when working in the registry.
The problem started when email links on web sites started opening in Thunderbird and continued to do so even after I uninstalled Thunderbird. I was unable to get Firefox to see Pocomail as the default again.
As a result I went in search of reasons and as a last resort, went to the registry. My preferred email client is an old version (2.64) and this is running in XP mode on Windows 7. Poco has been working fine for ages and I think the problem started when I downloaded Thunderbird to trial it then decided to stick with Pocomail.
The only problem has been that I can't get it back to be recognized for web links. I've uninstalled and reinstalled without achieving the desired result. When I click on a email link in a web page I get a message stating "Could not perform this operation because the default mail client is not properly installed."
Until I installed Thunderbird all was fine now I just can't get it to work correctly. Both Firefox v8 and Pocomail have been uninstalled and reinstalled twice but the same problem occurs when using Internet Explorer.
I'm attaching snips of the registry entries found under
HKEY_LOCAL_MACHINE\Software\Classes\mailto just in the hope that someone may be able to say if there is a problem there to be fixed.