https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/477216/PBS_dependant_guidance_11-15.pdf
Additional evidential requirement for dependants of PBS migrants whose last grant of leave was not in this capacity.
- If you are applying as a family member of a PBS migrant and your last grant of leave was not as their dependant or you are a child born in the UK, you will need to demonstrate your relationship.
Where you are married or in a civil partnership to a PBS migrant, you must provide your marriage or civil partnership certificate to support your application
Where you are applying as the partner of a PBS migrant but are not married or in a civil partnership with them you will need to provide documents to show that you have been living in a relationship akin to marriage/civil partnership for a period of at least two years - documents should therefore cover the whole of this period. Documents will only be accepted where they are from official sources such as utility bills or NHS registration:
Suggested Documents
Partners (including spouses and civil partners)
marriage or civil partnership certificates
bank statements
Council tax/ bills
medical registration
utility bills