Hi Kevin,
When BT set up your ADSL, they should have given you either a single or range of IP addresses, if they didn't then that means you're using dynamic addressing and I'm not sure I can help without checking more. If in configuration you give the router external ip address to your external network card and your internal card a local address, e-smith should sort out the sending traffic out through the router.
To modify the dns, again requires a question first, do you have virtual domains setup?, if you do then you can modify your dns map's but everytime you use the admin website to change a host details the changes will be lost and I'm not yet good enough at e-smith's syntax to tell you how to vary it.
What I did was go to /home/dns/var/named, where I found db. i.e. db.burninglight.co.uk, I used pico to edit the file and changed the www line to point at my external ip address of the website rather than my e-smith box. I just have to remember to remake the changes if I want to add a host.
Hope this help, any problems drop me a line.
Cheers
Mark
Burning Light Consultancy Ltd
mark@burninglight.co.uk