You're using the Microsoft Remote Desktop Protocol port (3389 TCP) on the external interface to forward to the PPTP VPN port (1723 TCP) on the Draytek so I'm not sure whether you've quite worked out how to connect things together.
The first things to say would be never
run Microsoft RDP live to the outside world and also never
use a PPTP VPN as it's obsolete and insecure. Those are hard and fast nevers, not sometimes nevers that can be ignored in some circumstances.
The Draytek supports several modern VPN encryption standards for LAN-LAN and mobile client to LAN but the easiest to get working for you is probably SSL VPN. Follow the instructions here
and note the following;
1) On the Draytek router under VPN and Remote Access >> Remote Access Control make sure that only 'Enable SSL VPN service' is ticked.
2) Under VPN and Remote Access >> Remote Dial-in User ensure that only allowed dial-in type is SSL Tunnel.
On the BT Hub forward an external TCP port to the Draytek's WAN IP address, Port 433 TCP. You can forward any
external TCP port from the BT Hub, it doesn't have to be 443, but that is the port number you will subsequently connect to with your VPN client from the outside world. 443 is the default TCP port for SSL but it means it will get scanned by legitimate and not so legitimate third parties almost immediately and may be targeted with username and password combinations in an attempt to guess your login details.
Choosing a random high numbered TCP port means it will take longer for external scanners to discover your VPN. If you keep an eye on your Draytek logs (I suggest you put a USB stick in the router for log storage) you will be able to identify persistent offenders and block their originating IP ranges with the Draytek's firewall.
Once you are able to connect the SSL VPN from the outside world to the Draytek, you can then use Microsoft Remote Desktop client to access your internal computer using its internal IP address - not the address of the BT Hub or the Draytek.
Does this make any sense? Shoiut out if you need more help and good luck.