Ah nevermind! Restarted both the VM box and router (again) and the issue has gone.
Prior to that, I ran smokeping internally against:
* 192.168.100.1 (VM modem)
* 192.168.0.1 (my router)
* external IP
I found that all of these showed the same packet loss/latency spikes, so clearly an issue with my Router (first hop) which i've ordered a replacement for before it dies completely 🙂