Given the internet problems I had at the weekend (see here, I went out at the weekend to buy a back up router from pcworld. In my haste, I didn't realise the N300 (this one >clicky< I picked up was simply a router and had no modem capability.

Sooooooooo.......thinking of using it as a repeater/wireless AP/range extender type thing so it can cover the deadspots we have downstairs (we have no plasterboard walls anywhere - all brick!).

Help on whether I can do this with the specific router?
I thought about those powerline things but would they work on extension plugs?

You'll need to disable the DHCP on the new router and connect from a lan port on the Draytek to the lan port on the new router.
In theory it should work but there's no guarantee as domestic routers are only designed to do one job