Sorry it's taken me so long to reply - shortly after I posted the original message it seemed to have sorted itself out so I kinda forgot about this post.
Until yesterday when it returned to very slow speeds for no apparent reason (we hadn't changed anything).
Result from speedtester.bt.com was test error - screenshot: http://imgur.com/K95Og
which is what I've always got from that site.
Result from thinkbroadband speedtest was 0.18Mbps - screenshot: http://imgur.com/uUEMg
(that's after trying the netgear router again - on the brightbox it was consistently at 0.06Mbps yesterday and earlier today)
I gave orange another call but they said it couldn't be raised to the fault managers team unless I tried putting it back in the test socket for 72 hours.
This is really driving myself and my family crazy as we all require frequent use of the internet and this is making that incredibly frustrating.
Any advice would still be greatly appreciated.