I thought the reference to "security" was to do with fraud protection. However, this contact page gives:
http://www.hsbc.co.uk/1/2/personal/contact Quote:Internet Banking Support
To set up a security number call 08457 400 004
To reset your security number call 0845 600 2290
A search of the database for 0845 600 2290 returns a verified entry:
HSBC 0845 600 2290 01226 261226 Internet banking
Try that and let us know how you get on.