| Thanks for the suggestions...
The marks are not caused by bird droppings or tree sap or any obvious pollutant. They are just where dust and general dirt had gathered. The dirt did seem to have formed patches rather than being evenly spread. I assumed this was due to the previously polished surface.
It could be acid rain though I live in a semi-rural area, but I guess there will be industry somewhere not too far away.
I may seek advice from the dealer before I do anything else. I'll look into the clay bar suggestion - I have no experience of it; perhaps it sounds more drastic than it really is. | 
