QOS

[#1]

Yesterday whilst downloading from steam and maxing out my line I caused the music streaming via Deezer to buffer 
Now I can enable FQ_Codel on my router to minimize problems with bufferbloat at the expense of a little bit of speed, but doing QOS at the CPE in the downstream direction is the worst place to do it (As you essentially have to drop packets to get the other end ot back off)
I thought the idea of the Plusnet QOS/Traffic management system is that streaming.etc was supposed to be prioritized at Plusnet's end to prevent this kind of thing happening.
Or is it the QOS treating the stream download as high priority/gaming traffic instead of a download?

[#2]

It's probably just that Plusnet gives most traffic the gold classification: http://www.plus.net/support/broadband/speed_guide/traffic_management.shtml
Streaming, web browsing, download servers - all the same priority.
Probably it's partly due to Plusnet deciding to give almost everything a high priority otherwise it wouldn't look good to people who don't understand how it works, although it's only the relative priorities that really matter.