Have set QoS up, in both directions, to reserve at least 40% for HTTP/HTTPS and 25% for FTP as quite a lot of files are regularly FTP'ed to Web Server. Despite this, when FTP'ing the whole setup just grinds to a halt. Speedtest shows .20 download and 0.01 Upload. Cancel FTP, and everything clicks back in to 'normal' working order.
It is a 2820N with firmware 3.3.3_232201 on. Any pointers would be greatly appreciated.

Did you ever get this issue sorted/resolved?

In the end, Yes.
Client was actually FTP'ing through HTTP/HTTPS so the FTP traffic I was looking for was, effectively, hidden!
Have amended QoS now to limit uploads by IP Address.

Excellent News.