Hi,
htaccess can be a pain and I will admit I usually manage what I am after with a bit of trial and error. Try the following, and if you have problems concentrate on the lines:
RewriteCond %{THE_REQUEST} .html
RewriteRule ^(.*).html$ /$1 [R=301,L]
I have added a redirect for index.html to root, and from non www to www and removed the last .html from the last list of _ to - redirects. Give it a shot, and keep that backup handy just in case. If no go, maybe one of the htaccess experts around can step in and have a look, I am not 100% sure what some of those other rules are doing to be honest!
<ifmodule pagespeed_module="">ModPagespeed on
ModPagespeedEnableFilters extend_cache,combine_css, collapse_whitespace,move_css_to_head, remove_comments</ifmodule>
<ifmodule mod_headers.c="">Header set Connection keep-alive</ifmodule>
AddCharset UTF-8 .html
<filesmatch ".(js|css|html|htm|php|xml|swf|flv|ashx)$"="">
#SetOutputFilter DEFLATE
#</filesmatch>
<ifmodule mod_expires.c="">ExpiresActive On
ExpiresByType image/gif "access plus 1 years"
ExpiresByType image/jpeg "access plus 1 years"
ExpiresByType image/png "access plus 1 years"
ExpiresByType image/x-icon "access plus 1 years"
ExpiresByType image/jpg "access plus 1 years"
ExpiresByType text/css "access 1 years"
ExpiresByType text/x-javascript "access 1 years"
ExpiresByType application/javascript "access 1 years"
ExpiresByType image/x-icon "access 1 years"</ifmodule>
<files 403.shtml="">order allow,deny allow from all</files>
# mod_rewrite On only needed once
RewriteEngine On
301 permanent redirect old underscore.html to new dash urls
redirect 301 /PRODUCTS http://www.timports.co.uk/kiln-dried-logs
redirect 301 /kindling_firewood.html http://www.timports.co.uk/kindling-firewood
redirect 301 /about_us.html http://www.timports.co.uk/about-us
redirect 301 /log_delivery.html http://www.timports.co.uk/log-delivery
redirect 301 /oak_boards_delivery.html http://www.timports.co.uk/oak-boards-delivery
redirect 301 /un_edged_oak_boards.html http://www.timports.co.uk/un-edged-oak-boards
redirect 301 /wholesale_logs.html http://www.timports.co.uk/wholesale-logs
redirect 301 /privacy_policy.html http://www.timports.co.uk/privacy-policy
redirect 301 /payment_failed.html http://www.timports.co.uk/payment-failed
redirect 301 /payment_info.html http://www.timports.co.uk/payment-info
301 permanent redirect index.html to folder
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^/]+/)index.html?\ HTTP/
RewriteRule ^(([^/]+/))index.html?$ http://www.timports.co.uk/$1 [R=301,L]
301 permanent redirect non-www to www
RewriteCond %{HTTP_HOST} !^(www.timports.co.uk)?$
RewriteRule (.*) http://www.timports.co.uk/$1 [R=301,L]
301 permanent redirect all .html to non .html
RewriteCond %{THE_REQUEST} .html
RewriteRule ^(.*).html$ /$1 [R=301,L]
I put the site through Screaming Frog (