@CandyCreeper although it is your son's disclosure which is the reason for the referral and the social worker's visit they will ask a few questions and take a quick look around your house as part of their assessment, if they come out.
They need to investigate when a child says these things in order to check out if what the child says is true .
It is probably easier said than done, as it's your life, your privacy, but the best thing you can do is be open and honest with them.
So for example, you have a mental health problem, but you are managing this by counselling tome medication. That would be seen as a plus because you are seeking a solution for your problem so that it doesn't affect your children.
If they do come to your house they will look for a reasonable standard of cleanliness/ tidyness - so no dog poo or black bags of rubbish building up in the kitchen, but there really is no need to get out the polish and plump your cushions. They may do a brief check of your kitchen cupboards to check you have food for your children but they are not expecting kale and quinoa. Just basic stuff , frozen chicken nuggets, chips, beans , a loaf of bread and some milk.
They are allowed to, and quite possibly will, request to see your DC medical records through school nurse/health visitor / GP. They cannot do this for your own medical history although they may ask questions.
Again I appreciate that this is your private life and this may seem an intrusion but please try to respond openly and calmly. I've noticed that your responses to posters on this thread have been very defensive and whilst you may have good reason to feel defensive this may be perceived that you are being difficult. If you are a private person who feels anxious about intrusion , if you let them know to begin with why you are finding the process difficult they are likely to be more understanding.