Ah so as I suspected it's passed on to further routers in the series. I believed it to be the software loaded onto the router is bloated somewhere rather than the physical hardware being an issue.
In my experience BT don't consider latency to be a real issue, which is to say worthy of looking at as a fault. But it really should be if it's bad enough that you can't just put it down to congestion (which is an issue that should have clear paths to getting it solved too).
Anyway that's my short rant on that, hopefully not detracting from the OPs problem. Seems like the latency issue was temporary in your case. Hope you get things fixed.
