TG582n major speed problems

[#1]

I've raised a help assistant question about this but Plusnet tech support seem to have gone to sleep, maybe someone here can help:
I started to notice really slow responses on web pages (taking 30s or more to complete) and started to run speed tests (BT Wholesale and speedtest.net). I'm on 80/20 and originally (last year I was getting pretty respectable speeds http://www.speedtest.net/result/2571376322.png) but last week it varied between 5 and 18M down and fairly consistently about 0.50 up (yeah, right!).
After being told to turn off, wait an hour, then connect directly to the modem I was seeing the old rates again: http://www.speedtest.net/result/3716247072.png
But connected via the TG582n: http://www.speedtest.net/result/3716257825.png
I've tried switching wifi off and disconnecting all other cables to the router apart from the PC doing the test to no avail. Generally speaking webpages take ages to load - particularly the speedtest.net page that is littered with ads and trackers (normally I run Chrome with Ghostery and ABP but was using a clean copy of IE and Opera to make sure that wasn't having any effect). I could hardly get the speedtest.net page to load at all.  
This is the most recent test result, still pretty bad upload speed: http://www.speedtest.net/result/3718076046.png
(edit a TBB test gave this http://www.thinkbroadband.com/speedtest/results.html?id=14091634051216915392 Is the TG582n clobbering http requests? TBB up seems fine. I just remembered I set a single website parental filter to stop my Now Tv box calling home and updating itself. Could that be the issue? I'll be home in an hour so I can test this theory.)
What next? Reflash the router - can't PN support do that remotely?  Throw it in the bin?
2nd edit - SOLVED (I think) by disabling "Web Browsing Interception"
before http://www.speedtest.net/result/3718263703.png
after http://www.speedtest.net/result/3718267814.png
immediately back to slow after i re-enabled it: http://www.speedtest.net/result/3718277248.png
Just to be clear, I had Web Browsing Interception set to Enabled in System Configuration. I don't remember setting this but I think this may have been set as a result of setting Address Based Filtering along with a web address in Parental Control.
It looks like this is already a known problem: http://forum.kitz.co.uk/index.php?topic=13224.15 has some speculation about singe threads being limited to ADSL speed but nothing conclusive. For the moment I'll disable it and leave the Now TV box off.

[#2]

Try a factory reset -- paper clip in the hole at the back, hold it in until the power light changes colour ( about 8 seconds).
You'll need to re-enter any custom settings.

[#3]

I think that using the parental filter does cripple the throughput of the modem, there's a thread on the forum somewhere.