(a) is there something mobile -unfriendly about sites beginning
http://webmail
There shouldn't be - it should simply be resolved to an IP address via DNS, irrespective of the name of the node.
But you could try entering
Login instead of webmail.ntlworld.com, just to check.
Did it (your webmail) ever work before with your Nokia 6234?
I'm ex-blueyonder, and my webmail page is
https://webmail.blueyonder.co.uk/ - note the use of https.
Perhaps http / https is something to investigate?
SD