Right everything now seems to be setup correctly, although,
1:- From external I get the shared ip page
2:- From my LAN I get my routers configuration page.
So just to reiterate what I have done.
NOIP setup - points to my dynamic IP.
NoIP account - differentdomain.servehttp.com
points automatically to my external dynamic IP Address.
My domain name registration - www.123-reg.co.uk
is registered with www.123-reg.co.uk
The www CName is set to differentdomain.servehttp.com
The @ CName is set to differentdomain.servehttp.com
redirected to differentdomain.servehttp.com
The dig www.mydomain.co.uk
now points to my external ip address. and says that the cname is differentdomain.servehttp.com
Hope this makes sense.
Again thanks for your time.