UK Car Repair: Catalytic Converters - servicing?, lean mixture, rich mixture

Question
My parents own am M reg Rover 214, my father has looked after the car for years now and is fairly good at all the basic mechanics, plugs, points etc. Of late the car has been playing up and is intermittent at both starting and running, sometimes will, sometimes won't. He is convinced it may have something to do with the Catalytic converter which he has never really looked at nor serviced. he says that at times, there is a browny discharge (smoke?) from the exhaust. Any advice or guidance would be appreciated, this is their only means of transport and they cannot afford to put it into a garage for repair.

Answer
How many miles has it been driven?  The brown exhaust is cause for concern, It could be the exhaust is rusting internally, and in fact, the "cat" may be causing it.  Or, it may be a sign of water in a cylinder.  I would suggest checking the spark plugs for colour, white shows a lean mixture, black shows a rich mixture and BROWN means there is water in the combustion process. A light grey is not umusual.