Question Author
Thanks SA & R for your quick responses on this matter.
I have not seen the mark on the windowsill of Viv's friend who has indicated that she has tried white spirit without any success. Raw bleach could be the answer and it is something that I would try, on the basis that it is unlikely to affect the PVC because the product is sold in plastic bottles. Furthermore, I recall having read somewhere that WD40 is capable of lifting woodstains from carpets; so is therefore a further possibility in this case.
Viv will pass onto her friend the suggestions.
Thanks again......Ron.