[Solved] Can she do this?

Hi all, looking for some advice.
This is my first post on here so please bear with me.

About six years ago, I was given custody of my son after the social services and subsequently the judge, deemed his mother not fit to care for him, he was two years old. I had not been in a relationship with his mother since she was pregnant with him.
His three siblings were also removed from his mother and placed in to care.
His mother was a neglectful and sometimes abusive parent who's actions were reported to the social services on several occasions. His mother was someone who made friends very easily and would leave her children with people she hardly knew while she did things like escorting work.
On [censored] sites, she openly invited strange men to her house where her children were present.
A psychological assessment concluded that my son's mother had a personality disorder and she was unlikely to change, even if undergoing therapy.
Since then, until recently, my son's mother has had contact once per week, for two hours, supervised by myself.

I, along with my parents have provided my son with a decent upbringing since the early court order. My parents have had to put their retirement on hold to care for him whilst I had to work. They have also played a massive part in his education. I couldn't have done it without them.

Earlier this summer, my son's mother applied to the court for more contact including unsupervised contact and overnight stays.
During the last four or five years, she has taken part in therapy and passed a few parenting courses.

Recently, the social services and the judge have deemed these qualifications good enough for her to have the contact which she has asked for and concluded that she is a changed person. I however, know that she is the same person.

Since my son has had more contact with his mother, he has become more cheeky and argumentative.
A couple of weeks ago, my parents were taking my son to get some new school shoes and when my father asked him to get in his booster seat, my son started shouting at my father in the street, saying that he didn't need a booster seat. My son heard this off his mother when she took him somewhere in a friends car without one.
Whilst my father was being shouted at by my son, he momentarily lost his temper and brushed my son's chin with his finger. Although the contact was only very slight, my son lost his balance and fell over, grazing his elbow.

On the day this happened, I dropped my son off with his mother and I went away for the weekend.
When I returned, I had the social services and the police at my door, making enquiries as to the cause of my son's 'injury' after my son's mother had reported it to them. They then went to see my father and although the police decided they were no longer going to pursue the investigation, the social worker said she needs to do a further assessment which she hopes to conclude quickly, however, she has now taken annual leave so the matter is still unresolved and my son isn't allowed to stay at my parents house without me there.

About a month ago, my son's mother asked of it would be ok for her to go on holiday to Poland at the beginning of September, I assumed that this would be ok as my parents hadn't made any plans and they would have been able to look after my son whilst I was working. Recent events have obviously changed this and my parents will no longer be able to do this.
I have told my son's mother that she will no longer be able to go to Poland as I have no alternative carers.
She has told me that she will be going on holiday, she will leave my son with her partner and as she has parental responsibility, she can leave my son with whoever she likes.
I have told her that this is unacceptable and that she does not have my consent to leave my son with a third party.
There is nothing on the court order that says her partner can look after my son whilst she goes on holiday.
I have concerns that this is not good for my son who needs routine.
Can she do this?
Thanks.