When I enter a command through the browser address line
E.g. http://192.168.0.52/cfg.spi?com=DO3;F
 it resolves to http://192.168.0.52/cfg.htm?page=con

and causes no change at the Webbrick.

I am confident that the ipAddress is correct

I have tried ensuring that that instance of the browser is logged in to the Webbrick.

What am I doing wrong? 

P.S.    I am running Webbrick v6.2.1206
 I usually use Firefox, but I get the same result with Windows Explorer.

You need terminator, a colon on the end, also not sure whether the semi colon is needed or would work in this case. So try

http://192.168.0.52/cfg.spi?com=DO3F:

See http://docs.webbrick.co.uk/WebBrick/CommandSheet6_00.pdf section 1.1