Quick question regarding LAN setup on a 2820, we have 4 sites using 2820s and each site is linked to the others via a VPN. Do I need primary/secondary DNS servers setup on the LAN side? The WAN side is taken care of by the ISP but the LAN side currently looks like:

LAN
MAC Address : 00-50-7F-xx-xx-xx
1st IP Address : 192.168.1.1
1st Subnet Mask : 255.255.255.0
DHCP Server : No
Primary DNS :
Secondary DNS :

The PCs at each site use the DHCP/DNS server that is running on the network so I have no DNS issues but just wondered if I would benefit from having DNS servers specified on the LAN side of the router and if so what they should be - local DNS server, ISP DNS server, DNS server of another DC (Via VPN) on the network, etc.

If you're running Active Directory then your clients should all be looking at one or ideally two AD DNS servers. If the DHCP was coming from the Drayteks on remote sites then use the manual DNS box under LAN to set the DNS to the AD servers. If however the VPN tunnel drops then you'll loose Internet lookups. In which case you may need to manual intervene.