I think I may have sorted it, a neighbour who is a builder gave me some hippo heavy duty trade wipes. No idea what they are but they smell quite solvent based. I have scrubbed it and there was an orange / brown residue on the cloth. Just tried with a hosepipe and it seems to be much better. Won't really know until it rains at night as that was when it was at its worse. Hoping that and the new wipers will have sorted it.

Hope you get it fixed. I had a previous car which developed marks on the screen (though not quite as bad as yours) and nothing I tried would shift them. In the end it was one of the reasons I decided to sell the car.

I had some oily / tarry overspray on a car once I think it blew in from roadworks. The only thing that shifted it was WD40

Just waiting for it to rain now so I can see if its ok, will keep the wd40 in mind if its not done.

Have you tried damp newspaper, always worked for me 

Try toothpaste, or Ajax / Bon Ami (I don't know what they call it in the UK).  Put it on with a damp cloth and use some elbow grease.  allow to dry to a powder & remove with dry newspaper