Has anyone tried the 3.6.6 firmware for the 2850? Having been bitten by the 3.6.4 DNS issue, I'm loathe to try it. However, I do get the occasional lockup with 3.6.3b, particularly if I use the built in VDSL modem.

Yes. Been running 3.6.6 since it's release.
The DNS problem is fixed so I can now run my own DNS server.
I'm using the internal VDSL modem and getting close to 80M/20M when checking the speed.
System Uptime: 64days

That's very helpful, thanks.
That gives me enough confidence to get it a try.

3.6.6 is nice and stable it seems, and syncing at the full rate (and speedtest.net gives the expected throughput).

The only thing I've found to be a problem (so far) is that LAN port 4 doesn't seem to work. Obviously, I've been using it as "WAN2" for the latest few months with the OpenReach modem, but now I'm using the built-in modem on WAN1 and have disabled WAN2 I was expecting it to work as a normal LAN port. Is there anything I've missed?

I hate replying to my own post just minutes afterwards, but this might help anyone else who encounters the same problem.

Basically, when I was using WAN2 and an external modem, I used the VLAN facility to isolate the LAN ports and SSID1 from SSID2, so that I could have a second unsecured but isolated wireless network for occasional guests. If you're using WAN2 when you set that up, IIRC LAN port 4 (P4) is greyed out so you can't add it to the VLAN. Not unreasonable, given thatn P4 is actually WAN2. When I switched back to WAN1, P4 is now selectable but not selected, and so wasn't included in my VLAN.

Doh!

The Lan 4 to work as normal Lan & not anymore as wan) you also need to disable it >wan> general setup