Just realised Ciaran that I have another site on Joomla 3.10.11 and PHP8.1 and I can get into those Articles fine - so I am now very confused. Just thought I would update you.

Basically, this is too little information. If the frontend is running and you can't get in to posts in BE, it doesn't seem to be the template.
Generally, PHP is 8.1. not recommended yet. There are still a lot of problems. Which is not to say that it could work.
You should first set the "report errors" to maximum and turn on the "debug mode", maybe that gives a decent error message. Then you should check all extensions if they work with php 8.1.
Possibly switch off certain extensions (modules, plugins from third-party manufacturers) briefly and see if the BE then runs correctly...

OK thanks for this which I will try. The reason I am trying PHP 8.1 is that my server host is saying that they are switching off 7.4 in early February therefore I need to move my sites to 8.1. When I switch on error reporting I get the following but then can't get into the backend to switch it off so I am stuck - any ideas how to get out of this? I also still havn't had a response to an email I sent to info@joomla51.com last Friday - is anybody monitoring these emails?

Deprecated: Joomla\Input\Input implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /home/nepalcom/http://joomla4.nepal-systems.co.uk/libraries/vendor/joomla/input/src/Input.php on line 41

Deprecated: Return type of Joomla\Input\Input::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/nepalcom/http://joomla4.nepal-systems.co.uk/libraries/vendor/joomla/input/src/Input.php on line 170

Deprecated: Joomla\CMS\Input\Input implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /home/nepalcom/http://joomla4.nepal-systems.co.uk/libraries/src/Input/Input.php on line 31

Warning: session_name(): Session name cannot be changed after headers have already been sent in /home/nepalcom/http://joomla4.nepal-systems.co.uk/libraries/joomla/session/handler/native.php on line 128

Deprecated: Joomla\CMS\Input\Cookie implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /home/nepalcom/http://joomla4.nepal-systems.co.uk/libraries/src/Input/Cookie.php on line 0

Warning: session_cache_limiter(): Session cache limiter cannot be changed after headers have already been sent in /home/nepalcom/http://joomla4.nepal-systems.co.uk/libraries/joomla/session/handler/native.php on line 235
Error: Failed to start application: Failed to start the session because headers have already been sent by "/home/nepalcom/http://joomla4.nepal-systems.co.uk/libraries/vendor/joomla/input/src/Input.php" at line 41.

Thanks I have now found the problem plugin and disabled it and I can now get into the Articles fine.

OK !
I could well imagine that you could get other problems, you should test everything.
I still have one question: Why don't you use PHP version 8.0?
P.s.: Think about an upgrade to Jooomla 4 soon... there are also a few things to consider!