No connection to Internet if wan link resets

I'm head scratching here and need some help.

I'm on a FTTP/FTTH link via BT in the UK. We are using an i5 PC running a snapshot version of OpenWrt as our router/public facing server (we need a module which is not available in the stable branch). We are currently running r22379.

Periodically BT does work in the exchange that takes out my line. In the last month or so, when this happens and the wan automatically reconnects, the internal network cannot reach the internet - pinging etc fails from local devices and on the router itself - but the world can still reach our webserver which is stored on the router. If I restart the network service or reboot the router, everything works.

Does anyone know what could be causing this, and what I can do to mitigate it without resetting the network (which triggers stuff with BT if it happens regularly)?