Just a further post to see if anyone has any inspiration - interestingly, I noticed an extra quirk this evening.
This is the hlsw ping graph for the community server:

and then the community server, after resetting my router:

The reset is clear to see as the long period of red, and then afterwards, the improvement can be seen also.
Interestingly, the arena server graph was the same (i.e. not elevated) before and after the router reset:

So it seems the problem only affects game traffic (as a normal ping/tracert would have not been elevated) and but indeed, only to a server that I have (presumabley) played on. I.e the lag only occurs when the traffic is going via a specific port.
The above, as well as the fact that when using the USB modem over the weekend, I didn't get this elevated latency at all, is making me believe more and more strongly that it's the router.
Now to convince Sky...
[EDIT: You'll probably need to click through on the pics to see all the graph, and hence the interesting part of the 2nd one]