Anyone know how to implement the equivalent of the Cisco "clear DF bit" setting for IPSec VPN with a Draytek router?

I have several 3300V routers and cannot find out how to do this. It's critical as our network has some packets which are too large to pass over IPSec VPN but have the DF bit set, so the 3300V drops those packets.

With Cisco you can tell the crypto to ignore the DF bit, I'm guessing there's a way to do this on Draytek models too.

I'm even willing to change to different model Draytek routers if there's a different model which can do this, it's so serious for us.

Tim