It is a newer type with a trim pot and switch combination.  I have since tried tuner cleaner which has resolved the issue.  Thanks everyone for your assistance.

Hi there, yes its a DC controller and it has Phillips screws.  I have taken a look at the combo trim pot and switch mechanism but no details on it.  I believe all I need do is replace it.   I will try mouser.com.  cheers.

I am having issues with the power pack for my layout product code 44212.  The trim pot is playing up, for example when I gradually turn the knob the train starts off fine, turn the knob a little more and train suddenly hits maximum speed..  Not good.  Turn the switch down and it is very erratic at low settings.  What I would like to know is where I can get a new trim pot and switch? 

Is this possible?  I am guessing I am up for a new controller.