I thank you Tarman - as if by magic .....
Your employer will have to declare the private use on your P11D form and you will have to pay tax and National Insurance on that amount of private use. Your employer will also have to pay Employer's National Insurance.
The value on which you get taxed will be 20% of the helicopter's market value
http://www.hmrc.gov.uk/helpsheets/hs210.pdf
Cheers
Whirls