[Solved] concern over ex new partner unsupervisied access

Hi apologies for length of post I am very concerned about my ex new partner beginning to gain unsupervised access to my four year old son.
Background: I spilt with my partner 3 years ago and obviously expected that at some point she would meet someone else. I have no problem with this as a happy mum means to me a happier son. I did have and still have very strong feelings of anxiety regarding him taking my roll over, becoming, his dad if you like, feelings that have settled over the 4 months she has been seeing him. (My son lives 40 mins away with his mum, grandparents and brother sister in-law and their two children. Her partner lives in Glasgow and is unemployed/part time/collage and visits 2 times a week I think.
I feel that the way she has gone about it has made matters worse. For instance I found out from my son that” mummy has a new friend...oh I was not supposed to tell you” my sons words… she denies this was the case and a misunderstanding. Other things like the new partner showering my son with presents like kites (symbolic dad and son thing) have also caused concern. The concern is that she appears to say one thing and do another consistently and generally deliberately confuse issues, not come clean which really adds to my anxiety. I have asked her to just be honest with me to no avail.
When I mentioned in a disagreement. Why does he not spend time with his own son which was playing on my mind she said it’s none of my business.
I wrote a 6 page letter to try to gently raise my concerns and did not really receive a reply but she did try a little to validate some points my concerns.
She eventually asked his permission to say why he couldn’t see his son was because his ex-had moved on with someone else and was stopping access because she he believes wanted to me on. This came as a bomb shell. I try not to judge but I feel I have a right to feel suspicious. I’ve watched numerous shows and talked to single parents and although it does happen it would seem that the norm is for a mother’s to deny access purely because she’s meet someone else and that the matter is always more complicate than that involving possible wrong doing on both sides stereotypically the mans. My point is she cannot be 100% sure his version is a true version and I cannot be 100% sure he is not telling the truth. But I feel it is my duty to make sure my son is safe and protected. On top of that, even though I try not to question her judgement I do know that practically every past relationship she has been in has ended in domestic violence. He did relay through her that he really knew what it’s like and had no intention of trying to be his dad. She also mentioned he was looking to go through the courts. I feel that I should know of his progress. If not why not?
Case in point: last week she informs me that out of the blue that he is looking after him while she goes to work and respond with confusion she says well pick him out earlier then. She then says okay its best you two don’t meet today he’s going to go hill working. My question is why is he not back in Glasgow at the lawyer’s office instead of looking after my son and going hill walking? I can see that this statement is borne from frustration , confusion but ( and this is why I’m writing I just been informed he will be at hers this week Monday to Friday morning and she is working Wednesday Thursday Friday and Saturday all day and night. Where is his son? Is the next “evolution” going to be he looks after my son on his own all these days?
She reassured me that he would not be used as a baby sitter while he is at work but that has changed which she did not inform me that this had now changed … under the circumstances I hoped that she would let me know of” evolution” as she puts it so in my mind the lack of consideration and communication is also the main cause of stress.
In summary over the last four months
1: very tricky misleading start
2: only been seeing this guy four months and he has gained unsupervised access
3: lack of consideration and communication
4: ever changing boundaries
5: sadly full history of domestic violence from her past partners
6: partner denied access to his own son

I believe it is still vital that I maintain good relation for my son’s sake one of the key benefits is I can use her cottage when she is at work which means I can see him in the week And would not have to sit down at the local village sports centre.
I am frankly at a loss I do not wish to alienate her or her new partner but I want to be sure my son is safe. Every time I talk or write to her she becomes highly defensive and I fear if I say anything now she will strip me of the workable mid-week visits

Is there a mediation service where we could both sit and talk?
Can I get background checks?
How do I tread gently? So as not to cause even greater heart ache and still he has access to my son

I am so all over the place I can’t work out what to do or even if I’m overreacting I just have a gut feeling that I have to make sure my son is safe
:unsure: