I am a US citizen with leave to remain permanently in the UK. How do I fill out the V2011B form?

I have lived in the UK for over 20 years and still hold my US passport. I have a number on my stamp in my passport as my verification of my leave to remain permanently in the UK but no official letter from the Home Office. Is it okay to just enter the number or would I be advised to send a photocopy of the passport page? The wording of the V2011B form asks for an expiry date - will I be advised to just enter 'Indefinite'?

Also, I am visiting friends in Hong Kong after the Human Brain Mapping conference in Beijing (June 10 - 14) and we may want to travel into mainland China. Should I apply for a visa that allows me to enter twice within 3 to 6 months (Section 2 of V2011A form)? Or would I need that visa (multiple entry) to visit Hong Kong in any case?

Thank you for your help!

Anne