I'm utilizing a Magento installation using the a measure checkout wordpress plugin, meaning I've very couple of pages that require HTTPS. I wish to control HTTPS search indexing and user access by redirecting the entire HTTPS version from the site except obviously for that secure checkout.

The secure checkout section is /onestepcheckout/ (as well as includes some sub pages of this, i.e. onestepcheckout/failure).

This is exactly what I've to date however the HTTPS isn't kicking in around the checkout page with this particular code, the redirect works in most other situation (i.e. I send index HTTPS to index HTTP) only the exception that's damaged:

#Redirect HTTPS to HTTP except checkout 
RewriteCond %{HTTPS} on
RewriteCond %{REQUEST_URI} !^onestepcheckout
RewriteRule ^(.*)$ http://www.mysite.co.uk/$1 [R=301,L]

#Require SSL on checkout
RewriteCond %{HTTPS} !on
RewriteCond %{REQUEST_URI} ^onestepcheckout\/?$
RewriteRule ^(.*)$ https://www.mysite.co.uk/$1 [R=301,L]

I'm also vaguely careful the /onestepcheckout/ doesn't contain secure assets and for that reason the best may require contributing to images and elegance sheets etc.