Following the useful suggestions from this thread, I changed my BT router and QNap NAS settings to speed up my downloads.

1. Disable UPnP in the router (BT Home Hub, Advanced>Application Sharing>UPnP menu)
2. Change the torrent port on the NAS from 6881-6991 to 16881-16991 (from Download Station on the NAS click “Set Config” and change “BitTorrent port range: to 16881 and 16991”, and turn OFF “UPnP NAT port forwarding”, change the download speed and upload speed to 512KB & 25KB respectively)
3. From router create a new Service for ports 16881-16991 and forward this service to your NAS. (Homehub, Advanced> Application Sharing>Create a new game or application. Fill in Name and choose manual entry of Port Maps then click Next. Leave Protocol as Any and fill in 16881 in the top Port Range box and 16991 in the bottom box. Then click Add. Then click the Application Sharing menu and select your new name game or application in the bottom left drop down and your NAS in the right hand drop down.)
4. In your NAS admin TCP/IP configuration, ensure you have a fixed IP address, your router’s IP as Default Gateway, and fixed IPs for DNS servers.