I wasn't able to use the 403 method because I wanted the maintenance page and page images in a sub folder on my server, so used the following approach to redirect to a 'maintenance page' for everyone but a single IP*
RewriteEngine on
RewriteCond %{REMOTE_ADDR} !**.**.**.*
RewriteRule !^maintenance/ http://www.website.co.uk/maintenance/ [R=302,L]