OK so it's not quite the windscreen, but a persistent smearing which is making night driving in particular rather hazardous.
Having cleaned the screen with soapy water (weak hot Fairy Liquid solution), rinsed, dried and then cleaned with white vinegar, it seems to stay clear for about a week. Gradually the contamination returns. It is only visible when it's raining, when dry the screen appears clear. It is most visible (and dangerous) when driving at night with the glare of on-coming lights.
The car is not parked under trees - in any case I would expect to see tree sap or similar in the dry. I do not tend to travel on major roads a lot - to pick up road contaminants thrown up in wet weather. No other colleagues are complaining of this - so not a problem caused by the atmosphere around work.
Although I haven't replaced the wipers they are in good nick. Removed and cleaned with Fairy, meths, white spirit, vinegar.
The key is needed to lift the bonnet to access the washer bottle, so I think it's unlikely this has been sabotaged.
The only source I can think of is the possibility of some break down of the washer bottle screen wash (Halfords possibly). It is not clear whether the rear screen is affected, being served by the same wash bottle. The smearing does not appear immediately on the screen after using the washers, but it's not reasonable to be focused on the screen too much while trying to drive. So at what point the problem starts is not obvious.
So I thought to syphon out the contents as far as possible, then use the washer pump to finally empty. Refill with hot water/vinegar solution and pump some of this through to front and rear screens to clear pipes, then syphon out remainder. Flush with clean hot water. Refill with water and screen wash.
Any comments/similar experiences and resolutions most welcome please.
Any better ideas on remedy or probable cause?
Thanks
Phil