I can't figure out how to do this at all - read the manual, sent a support request, read the guide on Draytek's website - I just get nowhere - the main issue being that I cannot figure out on the WLAN general setup page whether this WLAN setup is the AP810 as it will be connected to - or whether it should be the details of the router I'm trying to connect to. Their explanations are ambiguous to say the least.

Either way I've made attempt after attempt using every which method I can think of and just can't get anywhere.

I can get it to work as a repeater OR an access point if I tether via ethernet cable but that doesn't give me the range I need - I need to get it listening to my wifi router - and I just can't get that to work - but I fully admit I simply do not understand the instructions the user guide gives

If you want the AP to wirelessly connect to your existing Wifi and then allow clients to connect to it I think you'll want universal repeater mode:
http://www.draytek.co.uk/support/guides/kb-wlan-ap700-universalrepeater

There is also this guide which explains how to link a AP810 to a DrayTek router, but the AP810 side of the configuration would be the same if connecting it to another router.
http://www.draytek.co.uk/support/guides/kb-wireless-lan-universal-repeater