Hi got a little confused what Mode I should be setting my Ap900 to.

Not sure if I have a faulty AP as I have tried loads of settings, (I started from factory default each time) just cannot see it being this hard to extend a WiFi.

My setup options are:

2860vn-plus - ethernet cable to - AP900

2860vn-plus - WiFi to AP900

I will also want to use a couple of the ethernet ports on the A900 for devices to have connectivity back to the router.

The user guide is frustrating as it gives a few network diagrams but doesn't state which mode the AP is set to in that scenario.

I remember buying the AP900 because the 2860 could manage it, however I haven't found any guides for this as yet.

if helps
http://www.draytek.com/index.php?option=com_k2&view=item&id=2777:ap800-how-to-use-universal-repeater-mode&Itemid=293&lang=en
http://www.draytek.co.uk/support/guides/ap-900-managed-wireless
http://www.draytek.co.uk/support/guides/ap-900-troubleshooting
or have a look here if you find any user guide (from the 800 or 810 should be fine)
http://www.draytek.com/index.php?option=com_k2&view=itemlist&task=category&id=92:wireless&Itemid=293&lang=en