Since changing the channel from 6 to 11 to see if this would improve connection, it's actually worse. This is after I performed a Wi-Fi scan to see which local inits were in range and what channel they were using.

Some of my units don't see the main ssid automatically and although the wireless strength suggests 'excellent', actual usage is really bad. Surely changing the channel should not have had this level of affect?

Perhaps more possible reason ... is because some tools (including the one from draytek) only showing wifi signals from routers ... could be on this channel (11 in this case) signal from a cordless phone, or a baby monitor or a wifi hidden signal (draytek can do this i think) or a remote control of a unit etc ... most of them they do operate at same signals of the spectrum/range 2.4 G band but the free tools or in other words the basic tools will not detect them

i always using inSSIDer (just has little more details) but still will not show either signals from other devices unless we get the paid version and is expensive for me (and i don't really need it)

Another reason but less possible, could be according your environment such masonry,walls, location of the router etc ... channel 11 may not be the best ...

babis is correct, also look to broad spectrum sources of noise. Things like microwaves, power lines and high voltage machinery all play havoc with wireless signals.

There's a summary of 'everything' you can do to improve wireless LAN:

http://www.draytek.co.uk/support/guides/wlan-performance