I did a little more testing and noticed that with WAN down, clicking Inquire Time, on the System Maintenance, Time and Date option, did pick up the time from my LAN ntp server ok.
Changed the internet connection settings, so the WAN would never go up. Rebooted the router and it came up with a 1 Jan 2000, date. Didn't change in 10 mins.
My router was set to check the time every 30 mins. I did wonder if the scheduled checking was still working, even with the WAN down.
So changed to check every 30 secs. Rebooted the router and low and behold, the time was picked up from my LAN ntp server after 30 secs.
My guess is that WAN doing up triggers an intial sync, but the routine sync always runs.
Have changed my time checking to 1 min, so after a reboot my router will get the right time even if the WAN is not working.

Colin

That's fantastic news, I appreciate your perseverance Colin, I guess I was too impatient when doing my testing.

Thanks for your contributions HornbyP & Colin - good result.

I just tried this on an old 2830n - and yes, with a local NTP server configured, no WAN connections and an 'inquire' timer of 30 seconds, it worked

Last time I tried this, the 2830 never got a valid time - however - the LAN was a different subnet to the one containing the NTP server. I had a great deal of difficulty setting a static route for the 2830 itself (wifi clients were fine, as they got theirs via DHCP). It was probably, some aspect of that that tripped me up...