Try changing secure DNS over HTTPS (DoH) to "Off" or "Increased Protection" and in Connection Settings change "Use system proxy settings" to "No proxy".

Thank you for the suggestions but the problem still exists.

DNS over HTTPs was already off, changed to Increased Protection - restart Firefox, problem remained and set to "No Proxy, restart Firefox, and the problem remains.

If it helps --- I've just tried using a "Private Window" and found that it works but not a "New Window".

Try disabling your extensions.

Haven't got any extensions - and never have!

Does it still happen in Troubleshoot Mode?

Yes, the problem still occurs in troubleshoot mode.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Thank you for your continued support with this one. Refeshing the existing profile made no difference, using the Developer Edition, however, did connect to the website but it took a long time to do so, 10s of seconds, where as the BBC website would load straight away. Private mode on the "regular" Firefox loads quicker than non private Developer Edition. Using Private mode in the Developer Edition loads much quicker than non private mode.

An update... Although I can connect initially using Private mode or Developer Edition, if I then try and use the website (mouser.co.uk) by searching for a component, the problem returns with the Secure Connection Failure. :o(

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose, make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website, data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Try disabling any third-party antivirus or security software.

Apologies for the delay in replying, I have been travelling. The only anti virus running actively is Microsoft Defender, the security software I have is NordVPN but the problem occurs whether that is enabled or not. Having realised that same thing happens if I use Chrome - which then infers it is unlikely to be down a problem with Firefox or Chrome per se, - I uninstalled NordVPN entirely, restarted the laptop and the problem has apparently gone away on both Firefox and Chrome. I'll test for the problem over the next few days to see if it returns, then re-install NordVPN and see what happens and then raise ths issue with Nord.

Many, many thanks for your suggestions and support in this matter, it has been very much appreciated.