Base of the question, is parental responsibility (or lack thereof) over two children whose father died. Mother entered new relationship, long term but unmarried, new partner treated kids as his own, but relationship breaks down. I've covered what the mother is (or is not), entitled to in my view, and it is my belief somehow, that stepfather could possibly be liable for maintenance of children, under CA1989 definition of 'parental responsibility', despite it stating 'parent'. No agreements are in place.
Having a look through Jordan's publishing.