ILR parent of a British citizen

[Beth2k13]

I am about to apply for my ILR for parent having access right to his child.5yr route.
I see my child every 2 weeks as we don’t live in the same city.I have all evidence to show I still see my child but my child’s mum have refused to write a letter to show that I still see her as proof of contact.Is there any other way to get a letter for evidence that I still have contact with my child as the application requires one.