Weird problem that has me baffled.

>>> Maybe it thinks the /css is your css folder...
YES!!! 

I amended the following .htaccess ReWriteCond and it worked first time.

Code:

RewriteCond $1 !^(index\.php|images|css|robots\.txt|favicon\.ico|sitemap\.xml|Sitemap-static\.xml)

It's obvious once the solution is provided Note to self not to copy and paste scripts that I do not fully understand.

Many, many thanks once again for your insightful suggestion

[off topic]
I have had spasmodic server problems for about a month and eventually Apache2 showed the following warning:

Code:

System information disabled due to load higher than 1.0

The problem was resolved by a server reboot.

Meanwhile I searched and tried a tremendous amount of suggestions, none successful. After the reboot both Google and Bing Webmasters Tools started to recognise the server. Also all WebPage Test Sites started to work.

It was GWT that reported this particular error but not the solution

[/off topic]