You mention the pollen filter. It could be that the drains to the bulkhead vent well are blocked, leaving rainwater nowhere to go except into the car where it can damage some of the electrics, but you'd know about that because you'd get wet feet.

The most common cause of cars not starting in the wet is deteriorated HT lead insulation or a problem with the insulation of the coil packs. Another problem specific to the Focus 1.6 petrol engine is that the gulley on top of the engine in which the spark plugs sit is prone to collecting condensation water, causing starting problems and eventually rotting out a core plug. So have a look and try drying that out.