Urgh, this is giving me a headache.

I've got a 2820n pre-setup, latest firmware, and I'm trying to add an AP800 to extend the wireless...

There's no option for connecting the AP800 by ethernet, sadly, it has to talk to the 2820n wirelessly.

Furthermore, the 2820n and the AP800 must be able to take BOTH wireless clients (there are currently clients on the 2820n) AND Ethernet clients.

I've tried Universal Repeater Mode, I've tried WDS and it doesn't work... argh!

Help please, how is this supposed to work and what's the difference between WDS and Universal Repeater?

The 2820n has a SSID of a main, and an unused, SSID hidden visitors version (which I might turn off during this... ), the main SSID uses MIXED(11b+11g+11n), Mixed(WPA+WPA2)-PSK. Not using Mac Address Filter, but am using Bind IP to Mac, IP is 192.168.1.1, internet all works, and the AP-800 is on 192.168.2.1.

I've tried turning Bind IP to Mac off.. no joy. I'm also a bit confused by the Mac addresses... the AP-800 gives the same Mac for all it's connections on the web interface, but I know it uses 00, 01 and 02.... the first two are the default two SSIDs enabled, and 02 appears to be the mac it uses when connecting to the 2820n in repeater mode.

Not matter what, I can't get a ping between the 2820n and the AP-800, when I was using WDS it would say UP, then DOWN, then UP, then DOWN... I've put in the same security settings, SSID and channel....

The reason for all this is to get wireless to a Vista machine with a Vigor N61 dongle, at first I blamed either Vista or the Dongle (or it's drivers), but if I can't ping between the access point and the router that's not a good start.

Can someone advise please?

There is not much difference between universal repeater and AP WDS the only difference I can think of is that in WDS mode only traffic that needs to go to the router goes accross the WDS link but in universal mode everything is retransmitted even if it is not going to the router. So initially its easier to try universal repeater.
First you dont need to bind the IP to the MAC for the main router. You need to use the same subnet for all the devices.
First set the router DHCP server in LAN setup to start dishing out numbers with a start IP address of say 192.168.1.10.
Renew the leases of all connected devices to clear the 192.168.1.2 to 192.168.1.9 IP range
Then set your AP800 DHCP server to off.
Set the AP800 management IP to 192.168.1.2 and bind the mac of the IP in the router to 192.168.1.2 set the gateway address in the AP800 to 192.168.1.1 subnet mask to 255.255.255.0
Set the wireless mode, SSID and security settings to be identical in both router and AP.