I think I've got it - it's something on my wi-fi network; if I choose hop 1 the lag spikes are there already. So I can stop cursing British Telecom at least. Now to find what's buzzing my network! It's none of the devices that the router lists as connected...

Let me know if I'm misinterpreting, but if not, thanks for listening - sometimes it's easier to sort out a problem when you're not feeling all on your own smile