One of the things I forgot to mention was to set the BT router's DHCP to something other than 192.168.1.x. I use 10.0.0.x on my outside router. I'm assuming the settings of the Time Capsule are the same as my AirPort Extreme below.
Highlight the Time Capsule in AirPort Utility 5.6 and you'll see it assigned itself 192.168.1.1. Select Manual Setup. Click on Internet->Internet Connection, ensure it's set for Ethernet, Automatic, and Share a public IP address. On Internet->TCP/IP, configure IPv4 Using DHCP and you'll see the DHCP info it gets from the BT router, including the external IP address as it's outer one.
Click on DHCP and you see the range of addresses it will assign. You can either assign the DiskStation an IP address outside this range, or you can assign it a Reserved DHCP IP address within this range. End result is the same. On the NAT tab, leave Enable default host unchecked and check Enable NAT Port Mapping Protocol. Click the Configure Port Mappings button, which will take you to Advanced->Port Mapping.
If you aren't going to use a VPN:
Click + and DON'T choose a service. Enter 5000 in the Public and Private TCP Port fields and set the Private IP Address to the one you chose for the DiskStation. Click Continue, put DSM HTTP in Description and click Done.
Repeat for the following Ports/Description: Unless otherwise specified port is the same public and private (and you only need do now those ports you'll use). In Control Panel -> DSM Settings -> HTTPS Service, enable HTTPS and Redirect to HTTPS. Use HTTPS instead of HTTP where possible.
Port 5001 - DSM HTTPS
Port 5005 - WebDAV HTTP
Port 5006 - WebDAV HTTPS
Port 5007 - Alt Web Svcs
Port 7000 - File HTTP
Port 7001 - File HTTPS
If you'll run a public web server, Photo Station, Web Station, or Mail Station. Make sure to add a deny all to all robots.txt * to /web
Port 80 - Web HTTP
Port 443 - Web HTTPS
If you'll run a mail server:
Port 25 - Mail SMTP
Port 587 Public to Port 25 Private - Mail SMTPS **
Port 109 - Mail POP
Port 143 - Mail IMAP
Port 993 - Mail IMAPS
Port 995 - Mail POPS
If you'll want remote access to the Linux command line or SFTP server, map a high numbered port to port 22 to keep brute force attacks on port 22 from eating up your bandwidth. See Method 3 here
Port 22022 Public to Port 22 Private - DS SSH
More ports here
* Contents of deny all to all robots.txt
** For submission, add the following lines to the bottom of /usr/syno/mailstation/etc/main.cf
smtpd_enforce_tls = no
smtpd_tls_loglevel = 1
smtpd_use_tls = yes
You'll probably find the Time Capsule like the AirPort Extreme in that it does loopback so you can use your external DDNS or domain name from inside your LAN.
Hope this helps.