Secret Ballot?

If - as we are always told - we have a secret ballot then why does the clerk cross reference our electoral roll number on the counterfoil of our numbered ballot paper?

I'm trying to make a one woman stand against this creepy version of a secret vote. I'm going to go in claim my ballot paper then persuade everyone else behind me in the queue that we should all swap papers around before we vote. Then our choice will be secret . .

Here is an 'official version' of why the electors registration number is noted on the counterfoil -
http://www.walsall.gov.uk/index/council_and_democracy/elections/voting_faq.htm#7347

Well - they would say that wouldn't they!