I've had to admit defeat and return the router as despite checking and confirming that the config was correct with Draytek technical support the DHCP lease renewal caused loss of access (but not VDSL connection) on the hour every hour.

They could give me a definitive reason for the issue so put it down to a localised Sky issue.

I have got hold of an Openreach modem and think I will get one of the Asus routers to go with it.