Remove trailing .html rewriterule creates 404 error - apache

Question

I have modified .htaccess to remove trailing .html from my pages. htaccess looks like this:
RewriteEngine on
RewriteBase /
RewriteCond %{http://www.mydomain.co.uk} !(\.[^./]+)$
RewriteCond %{REQUEST_fileNAME} !-d
RewriteCond %{REQUEST_fileNAME} !-f
RewriteRule (.*) /$1.html [L]
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^.]+)\.html\ HTTP
RewriteRule ^([^.]+)\.html$ http://www.mydomain.co.uk/$1 [R=301,L]
I also added the following at the top of the .htaccess to set up a custom 404 page and stop indexing:
ErrorDocument 404 /notfound.html
Options -Indexes
The rewrite rule works perfectly. So does the anti-indexing. However the 404 page gives me this error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster#mydomain.co.uk and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
If I remove the rewriterule then the 404 page works fine. I don't know a lot about this kind of stuff and was wondering what to do to be able to get both aspects to work correctly? Can anyone help please?
(PS: I read this answer and think it might apply to me - but didn't understand it at all.)

Answer 1

Just wanted to let you know I found a solution for this:
ErrorDocument 404 /notfound.html
Options -Indexes
RewriteEngine on
RewriteBase /
RewriteCond %{THE_REQUEST} ^[A-Z]+\ /([^/\ ]+/)*[^.\ ]+\.htm\ HTTP/
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^(([^/]+/)*[^.]+)\.htm$ http://www.example.com/$1 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)/$ http://www.example.com/$1 [R=301,L]
RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^(([^/]+/)*[^./]+)$ /$1.html [L]
Works great!