I thought voting was secret. Why does the clerk write my poll number on the ballot paper counterfoil?

It is a legal requirement that poll numbers are written on the back of the ballot paper counterfoil. The procedure exists to detect and prove any possible abuses or fraud. At the end of the poll, the counterfoils are sealed in a secure packet. The packet of counterfoils is not opened at the count.

At the end of the count the counted ballot papers are also sealed in a secure packet.

After the election the sealed packets are delivered to the Clerk of the Crown (Parliamentary Election) or held secure within the Council Offices (local election). The sealed packets can only be opened by an Order from the High Court or County Court provided that the Court is satisfied that an Order is needed to help prosecute for an election offence.

The procedure is there to protect the integrity of the democratic process and not to undermine it. Your vote is, therefore, secret.