Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first.
How old is her son and do you have any children together?
When did you ask her to leave and what reply has she given?
Since you are not married and the child is not yours, and she has made no financial contribution to the house your girlfriend has no financial interest in the property.
However she is entitled to "reasonable written notice to leave" and given that there is a child involved this should be at least 28 days - possibly more.
provided things can remain civil between you
I hope that this is of assistance - please ask if you need further details
No just a polite letter asking her to go by a certain date and saying that after that time you will change the locks is fine.
Keep a copy - and if she will not acknowledge it take a series of time and date stamped photos of it taped to a kitchen cupboard or somewhere similar.
You are most welcome I hope all goes well