I've actually solved the remnant problems I had after updating to 10.9.2. This is unlikely to be relevant to most of you here, but just in case it is:
My remaining problem was that although pings to the router had become reasonably consistent since 10.9.2, web-browsing to a few sites still stuttered or downright stalled. This only happened on my MB -- other devices (a PC, android phone and an iPad) were all fine. It was oddly inconsistent, in that the same sites worked fine on the MB on some occasions but not others.
The solution turned out to be to stop my modem/router from proxying DNS lookups (ie. instead letting DHCP justreturn my ISP's DNS IPs to clients). Actually I'm surprised I had forgotten to do this, as I've frequently found DNS proxying on consumer-grade routers to cause problems, so normally opt to switch it off.
I don't understand why the MB was the only device on my network that had DNS lookup difficulties as a consequence of the proxying, but it's definitely reproducible, so I'm satisfied this was my final remaining issue with the MB's networking. All is working smoothly and as expected now.