Thanks Ray. Have spent the last 3 hours trying to work it out and have narrowed it down to the htaccess file
I have got
#Redirect non-www urls to www
#RewriteCond %{HTTP_HOST} ^shillingstone-railway-project.org.uk [NC]
#RewriteRule ^(.*)$ http://www.shillingstone-railway-project.org.uk/$1 [L,R=301]
I have used this on many sites and they all work absolutely fine but as soon as I uncomment them on this site, the login screws up