OK, so away from PC for an hour, returned and I'm getting 420mbps again. I didn't get the chance to test the router/modem modes. It appears that between 6.30pm and 8pm for 2 days running the speed has dropped. I'm wondering if there's simply some congestion on the network.
At least I'm happy it's returning to something like normal. (and lets be honest, 100mbps is still enough for 99.9% of home internet use - it was my steam library updating that seemed slow, but it's still miles better than anything on ADSL or FTTC)