OHMYGOD I am a grade 1 pillock:
All sites hosting our mail servers have Zen as their ISP but where I am now it's using BT - so I guess it's not a good idea to try and use Zen's server from a BT account - mind you some kind of error return when I tried to connect might have helped!
Nigel
PS: Thanks Mike - I've just seen your separate email about this - I think I beat you by about 2 mins!