wrote:

Looking at the IP addresses which seem to be causing the long times in traceroute, they seem to be either BT ones or EE (my ISP) ones.

do you get *any* non-slow hops once it leaves your network? If not,
it's likely just to be dodgy DSL connection to cabinet or exchange

does your router give you any line stats? tried reboot router? noise oon
phone line? try calling your own line from mobile

leave pingplotter running for a few hours to try and spot patterns

BT and EE are one and the same ...