Did the sun come out after the rain? Maybe it has been scorched like with a magnifying glass?

Did you have hail? They could be little areas of damage from being hit at high speed by sharp lumps of ice. I note that most of the marks are short lines, orientated in about the same direction.

Thanks for the prompt replies. I think it must more likely be the hail, as the sun has been in short supply round here just lately!