can you try speedtest.net and see what the upload result from that is first?
I would make sure QoS and rate limiting, DoS protection, etc, is switched off on the Draytek.
Also, are you running the latest firmware?
Also, consider a reset to factory defaults on the Draytek, you'd be amazed what this can sometimes fix.
Unfortunately the Draytek 2850 is a little bit of an "odd one", they didn't produce them for very long.. I would definately contact Draytek UK support though if the above suggestions don't work, it is still considered a supported product.