[ubuntu-uk] Need advice re Super+Tab in Compiz

[Mac]

I've just upgraded my Dell box from Feisty to Gutsy;  it has a Radeon 
x300 graphics card, so I needed to install xserver-xgl to get Compiz 
Fusion working.  Advanced Desktop Effects working fine now... Except 
when I enable Ring Switcher or Shift Switcher, the key bindings 
involving the 'Super' key don't work, which means these plugins don't 
work at all.  (Alt+Tab works as it should for the Application Switcher 
plugin.)

And I can't change the key bindings for Ring Switcher and Shift Switcher 
- my attempted edits in the relevant section of the plugin options don't 
work.  There's just a system beep when I press the 'Super'+Tab or 
Alt+Tab;  but I can enter other useless characters, such as letters, 
Shift, <, and similar.

When I enable one of the other 'Switcher' plugins and try to use 
'Super'+Tab with say, a Firefox screen or a Thunderbird screen active, 
it seems just to select the next field or item (e.g. text of hyperlinks 
on the page in Firefox).  So it looks like there's some sort of key 
binding being set elsewhere (Gnome?);  but I've no idea where to look, 
or how to safely change this behaviour.

After hours of unproductive Googling, I was wondering if any of you good 
folks have come across this -- and fixed it!

TIA

Mac