I am certain nothing else happened. I just opened the template file in Visual Studio made a change to the text and saved the file then the problem started.
I have carried out your suggestion and followed steps 1, 2 and 3 but nothing obvious is shown - there are many items classed as Verbose but no Errors or Exceptions. The only Warning is as stated earlier "Console 'key5362938088057064' has a last
known change numer of 21, but server current number is 1"
Yes I tried restarting the browser and the website.
Unfortunately, I need to get the site working asap so I'll have to restore an older version and work with that. Thanks for all your help mawtex.