To be safe, if you have received any bonus shares on or after 20 September 1985, keep all the documents the company gives you.
For any bonus shares issued before 1 July 1987, you need to know when the original shares were acquired. If you acquired them on or after 20 September 1985, you will also need to know what they cost. Flowchart 1 in appendix 3 summarises the different rules applying to the treatment of bonus shares.
Keep a record of any amounts you paid to acquire the bonus shares and any amounts taxed as a dividend when they were issued.