can you confirm the follwoing
www.xxx.org.uk should point to 82.96.92.xxx and
mail.xxx.org.uk should point to 217.146.123.xxx
note: this will need to be checked externally as when you r on the LAN pinging the www address and the mail address will give you results in the local range - with non-routable IP's - ie 192.168.0.1
are you using 2 separate connections externally (physically)? if not then the port forwarding suggestions on your router are definitely where you need to be looking.
if you are on two separate connections, and both pinging mail. and mail collection is working as expected from the WAN then it might simply be that user-manager is not setup to be available externally. isn't this the default?
have you tried making a (or do you have any existing) externally available ibay? or you could try /webmail if you have this available externally. if this is available externally from mail.xxx.org.uk/testibay then the user-manager issue is specific to this page, and isn't dns related.