Hi Tara,
Firstly I just want to thank you for taking the time to give such a prompt and detailed response - it is much appreciated.
I see someone else from the UK a few months ago had exactly the same error as me but they were using a different provider.
https://wordpress.org/support/topic/error-503-backend-fetch-failed
The only access I have to the site is through the cpanel that Namesco provides and I can see they have reset the .htaccess at least 6 times now as well as resetting the permissions.
I will wait until Tuesday before following your advice as I do not want to conflict with anything that support may be doing.
From looking up the FAQ's on this site I know I can disable the plugins by editing the sql table wp-options.
I do not know what the default theme is (is there a way of finding out ?) but I can certainly rename the current theme which was installed by someone else early in September and I have been working with that through the Dashboard.
Prior to installing this theme I was never asked for FTP credentials when doing updates as I had inserted the information into wp-config.php
On the 21st October my host changed from ZEUS to Apache and every thing seemed to work as normal until Thursday when attempting to do updates.
I will do more research over the next few days and report back here.
Thanks again.
Ray