demi in the nicest possible way that's the biggest load of bollocks I've ever heard. Child maintenance is not based on new partners income, it's about providing the child with the same level of upbringing they would have had if the parents were still together. If they were still together then I wouldn't even be remotely contributing to her upbringing so no the new partners income isn't relevant when it comes to child maintenance.
It certainly used to be sort-of relevant. Not in terms of exact monies, but relevant.
For example my ex earned £2000 a month. His rent was £700, council tax £100 and other essential bills £200 meaning his outgoings were £1000 per month. He was then liable to pay x% of £1000.
However the fact that he lived with a wife who had a reasonable income meant it was fair to assume that the whole £1000 of the essential bills of their joint home (in their joint names) shouldn't be all down to him. In our case they said that he should be liable for half of the essential bills which meant he had to pay x% of £1500 for the girls rather than £1000. Obviously I've rounded amounts (it was almost 14 years ago!), but that was the general gist at the time.
It wasn't that her specific income counted toward the maintenance, it was that it was reasonable to expect her income to pay toward their joint expenses which meant he was assessed on his free income after that.
What happens if a man moves in with another woman and then gives up his job to become a sahd for a second family
Then his first family get nothing. Not even the £7 per week that someone claiming benefits would be due to pay.