Update (if anybody interested)..

Tried a few more things suggested by support...
- reflashing the bootloader
- reloading (the same) firmware 3.3.7.2
- reloading the complete router config by retyping (not using config file)
We got nowhere.

Grabbed a few sys ver dbg's and put syslog on and sent to support and they think issue in the WCF.
Turning WCF off completely seemed to stop the regaular restarts (but not ideal).
Now Support have issued me a beta firmware - beta_0414 Firmware Build Date/Time: Oct 24 2012 23:19:32
Loaded this and seems to be more stable.... not had any restarts yet in 3 days but UK support unable to tell me what changes there are in this firmware yet.

Will keep you posted...

Thanks for the update (wich more people would do that!). I think support won't know what has been changed as
it's likely code written by the programmers, which wouldn't be useful to know!

OK final update...

The 'beta' firmware that Draytek provided stayed stable so seems to have fixed the serious bug that affected the WCF. However, the beta firmware did not have the correct UK modem code. Now, for whatever reason, Draytek can't (or won't?) supply this fixed firmware with the UK modem code and I have to just hope that all my sites work fine and dandy with the 'international' modem code if I want my 2830s to stop restarting every hour.

There is a word up here in Scotland that sums this up nicely.... PISH!

The 3.6.3 firmware is available for non-wireless, single and dual band wireless routers, seems stable so far, also comes with IPv6 support.