Pete. well obviously you are getting wireless via the camera and external flash.
Are you selecting the onboard flash + external icon in the flash menu? Also are you altering the Flash mode setting from "EasyWireless" to "CustWireless". This setting should allow you to alter the ratio between onboard and external, but will not allow control over the individual power settings.
I'm not 100% sure regarding the 600D, but I don't think you can alter the individual power settings of your external flash via the camera menu. A simple procedure on the 7D.
These 600D setting are quite alien to me, as the 7d menu is totally different.
I've just downloaded the 600D manual so will peruse that for a while and see If I can come up with something.
Before doing so I will have a play with my 7D and yongou flash guns which incidently cost just over £100 each and are almost equal in design, build and specs. The only downside is they will not act as master.
I've not used my camera or strobes since August as first my daughter was in hospital then my wife. All is now ok so I need to get back up to speed on my photography.
Will look in a little later on to see how you get on with your flash problems and after reading up on the 600D may come up with some suggestions
Harry
Edited. Wireless flash is only available in the following modes P, Tv, Av, M or A-DEP. Also you may find these vidios helpful.
http://www.cnet.com.au/how-to-set-up...-339325393.htm
http://www.youtube.com/watch?v=pQIyPWGPp5A