The URL you are using in sharer link is
http://www.mydomain.co.uk:80/blog/category/page-title
and the URL at og:url is
http://www.mydomain.co.uk/blog/category/page-title
i.e. there is a difference in both URLs (:80 in the URL at sharer link), so it may cause issue.
Try to use same URL both place. May it helps.