My Time Capsule insists on being the DHCP server, but I already have one I have today joined the 21st century and started using FTTC, leaving aDSL behind as a fond memory. The modem that my telco provides presents a cat5 socket that my PPPoE capable router plugs into. 

My router (currently) is an Apple Time Capsule. Not an obvious choice as a router/firewall, but I had it already and it does handle PPPoE at the cost of one cat5 socket. 

The downside is that it insists on choosing it's own IP address (from one of 3) and being the DHCP server. I happen to have my own DHCP server and DNS server with a small DNS zone that I'd like to keep using, but the Time Capsule doesn't give me a means to achieving this that I can see.

Can anyone advise how I can configure the TC to use a static LAN-side IP address, and not to be the DHCP server, while at the same employing NAT to share the single IP address I get amongst my machines on my LAN.
Time Capsule can only handle PPPoE if it is also acting as the DHCP Server. You will need to get a different device, or allow the Time Capsule to be the actual DHCP Server, turning off that function on your provided modem: http://support.apple.com/kb/PH5104