After this thread I took great care to look very carefully as I went to vote just now.
http://www.theanswerbank.co.uk/Society-and-Culture/Question1417910.html
There was NO marking of the voting slips. The entire 'book' of slips that mine was taken from had the same number printed on each slip and counterfoil, that number was 41. There were other unused 'books' of voting slips ready to be used, the next book had the number 42 on it.
No serial number on the slip, I looked closely on both sides.
I still an certain that in all previous elections the situation has been the same. I have never seen a serial number on a voting slip yet, just the station ID number on the books of slips.
Anyone seen different? Let us know.