I had that problem a couple of weeks ago but the local car valeters got it off very easily using one of their 'wipe on wipe off' products.. You might find one locally.
I am told it is a product called Tar and Glue Remover, by a company used by the trade known as Trafalgar Cleaning/valeting Products.
He does not know if it is 'trade only' but it is very corrosive and has to be applied strictly according to the instructions. He warned me that if used on glass which contains any elements then any 'scrapping' of the glue is a no-no. Over elements it is rub using a cloth only. It has to be used quickly!
He does not know what quantities it is sold in as he only buys it by the 5 litres(?).