My opinion is although you have overnight staying visits and thats very good in the grand scheme, for a baby so young frequency is very important
I wouldnt write to her yourself as that might be inflammatory? I dont know.
Id attend a MIAM asap which are free and for legal aid still exists for ongoing mediation if you're eligible by income. I wasnt; she was....the ywrote to us both inviting the opportunity to talk.
....I agree, attend the MIAM and leave the Mediator to contact her directly and ask her to attend....she is more likely to take an official request more seriously, where a letter from you may not have the same impact.
If she is receiving benefits, even though you may not be eligible for legal aid funding, you will also receive the first appointment (the MIAM) for free.
If you can sort the issues out without court intervention that would be better for you all in the long run.
Good luck