One server unable to see another on the network

This is a bit of an unusual problem. I'm not too bad with networking but this has me stumped!

Here's the story:

The accounts dept moved out of the head office to a new office. A new server was bought, mostly to host a Sage database but also for future needs.
This was installed with Windows 2003 Server as a domain controller on the same domain. I set it up originally to connect to the domain controller in the head office via VPN for synchronisation.
This all seemed to work ok.

A few weeks ago I persuaded the bos to let me buy 2 VPN routers (Draytek Vigor) to establish a LAN to LAN VPN connection between accounts and head office via broadband.
These were bought, installed, setup and VPN established no probs.

The problem:

The accounts domain controller cannot ping the head office domain controller. It can ping every other PC on the head office network except for the DC.

The Accounts PC's can happily ping the domain controller plus any other PC in the head office over the VPN link.

So what the hell has happened? I have gone through the DNS entries and it did have an old entry for an IP address it gained when it used a VPN dialup but I deleted that. Its even been rebooted since then.

I cannot ping by name or by IP but a PC on the same network can happily ping by name and IP.

Any help?

Head office Network
192.168.1.x
255.255.255.0

Accounts network
192.168.10.x
255.255.255.0

Thanks.