Belkin Play N600 and BT Broadband - Help!

After many years of loyal service, our old Belkin router gave up the ghost. As I'd had no problems with it over the years, I decided to stick with Belkin and went for the Play N600, which seemed to offer a good range of features.

However, as you're no doubt expecting, it's been a tough time trying to get it to work properly. Out of the box (or on a factory reset) we're getting about a 0.1 Mb download speed (as measured on speedtest.net). We've gone through all the usual stuff, including:
  • Measuring the best MTU and setting the router to that setting
  • Switching to a channel not being used by other routers in the area
  • Switching off the 5GHz and sticking to the 2.4
  • Switching the bandwidth to 20MHz rather than 20/40
The last two seem to offer some improvement, going from about 0.1 to 0.3 - 0.5 Mb, but we're on an 8Mb line and were getting well over 1 Mb on the old router.

Phoned BT and they did a line check. They discovered a fault and the supposedly fixed it. When we told them the problem was still there, they sent an engineer who said it was either our laptop (we have the same problem on every device in the house), or the router (a possibility, I guess) and that we should be using the BT Home Hub instead. Now, I've used them in the past and have no intention of going back, but they're sending us a free one so we can at least test it.

In the meantime, any ideas?