How do I block the radio control on my digital Acctim clock?

I have a radio-controlled clock-specifically, an Acctim 74057SL (a href="http://www.amazon.co.uk/Acctim-Stratus-Radio-Controlled-Smartlite/dp/B002I9LYHE)." rel="nofollow"http://www.amazon.co.uk/Acctim-Stratus-R…/a It automatically sets itself by tuning in to a radio broadcast that transmits the current time. However, this method sometimes produces unreliable results (perhaps there is interference?), so I'd like to disable the radio control mechanism. Does anyone know what switches I have to press? Or maybe I can cover up the antenna?