Must have had a big bowl of Stupid-Ohs a while ago.
For the past few weeks I'd been noticing that our DSL internet was really slow. I couldn't watch Youtube videos without buffering every 30 seconds or so and whenever I'd run a speed test I was getting a maximum of about 1.5 of the 5Mbps I'm paying for.
Having just gone through an episode like this a few months back, which resulted in a modem replacement, I wasn't very ambitious to go through the whole trouble shooting process again.
Tonight I was poking around with my router settings and noticed that the QoS upload and download values appeared swapped. I switched them back and PRESTO, I got full bandwidth back
I feel so fawking stupid atm.
Generally unless you know what you are doing QoS is best to be ignored