Certainly a scare tactic, they cannot give the child to someone who does not want them, if they are aware that the father has stated that they do not want the child then they are basically telling you that they are willing to give your child to a household where they are unwanted and thus more likely to receive inadequate care. If they get you to go to this surveillance unit then they will be trapping you, another method for them to get between you and your child, also a method for them to try to wear you down and avoid you being able to reach out to others, them pulling this seems to equate to panic mode. Because if they can use those psychological evaluations to discredit you, then the public will pay not mind to your story and discard the facts as a grudge or ramblings of someone who is unstable when, honestly, you seem pretty damned stable to me and from reading this thread seem like someone who'd be a great dad, it says a lot with you still being determined to keep and care for your child even after being run through this mill.
All I can suggest is keeping written, and when possible audio/video record, I don't know if you have a voice recorder or camcorder, but by your social worker entering your home aware that you wish to record the visit, he/she is consenting to all interactions being recorded, if they refuse to allow you to record then you refuse to let them into your home, that is your private property, you have every right to record within your home, and if they try to refuse that then they are proving they have something to hide. They only need to be informed that entering the premises means they are consenting to being recorded, a simple sign on your door would suffice as a warning. If they're trying this hard then they have themselves set on taking your child, there may be no amount of behaving and jumping through their loops that will sway them from it, but the court can, but you have to have evidence otherwise it'll be your word against theirs, and a social worker's word is normally taken a bit more highly than that of the person they're hounding unless there's solid evidence to prove otherwise. This also includes saving messages they leave on your voice mail, letters they have sent you, all that, it's all evidence. I'm not a lawyer or anything but I've tangled with social services before, thought this was in the US, still, those are the things that saved my skin, I'm suggesting them because, maybe they'll help you.
I hope whichever path you takes works, and I hope they don't manage to take your little one from you, as someone who has two children they had to give up I know the pain of not seeing your little one every day, of not knowing if they're alright or not, I wouldn't wish that pain on anyone who had the potential to be a good parent, be they mother, father, either. I wish you the best in this, reading this thread you seem to love your child even though you've not yet met them, and that's what a good parent does, that's how a good parent feels, that shows you're be, not a good dad, but a great dad. I hope that the courts see that, for your sake and the sake of the child you're having.
That came out a lot longer than I expected, my apologies.