We have several clients using guest network with 810's and 900 AP's

All of them have two network cables with the second one providing the guest network so its isolated.

On the last chat session Draytek indicated that its possible to do this with only one cable..

Anyone done this - if so can you give me any details ?

You need to you Dot1Q VLANs and tag on the separate VLAN you are using
The port between the Router and the switch needs to be a Trunk. It will then carry both tagged and untagged packets