Anyway yesterday evening I decided to put my 2820 into bridge mode to get away from the fact I was using double nat. Instantly I did that the 2920 said no connection.
After a lot playing around I discovered I had to forward the first ip subnet under dhcp server settings. Bingo got a connection. Back to the draytek 120 and no option to do this. Back to square one.

Got it working and it was the arp option. However the instructions say that it shows connected and no internet access. Actually it shows no connection on the wan you're using which is why I didn't try that option. The 2820 in bridge mode partial worked without arp not all ip address's filled out so switching it to arp fixed that. I also had to forward the first subnet from the 2820 before everything played nicely.
However this now leads to another problem of the 2920 not falling over properly when wan1 keeps the connection but doesn't allow any data to flow. Using ping caused the the router to go hi I've got no connection really falling over to the other wan.With arp it just sat there saying your connected.