Non European tenants

Other

I have heard that landlords can be held responsible if they have a tenant that is illigal.
My problem today is i have a Nigerian couple who i have rented to for a couple of years and do not have copies of their passports or other id.
they have reconmended to us another couple whos passports are running out one has a work permit for another year the wife has an entry clearence but it does not have an end date and no work permit although she is working. This has made me worry about the first couple what am i entitled to ask for ?