Your not being harassed, the social welfare office is doing his job and making sure that you are a single parent and not just claiming to be one so you can recieve opf. Why did you lie in the first place? If you really are a single parent and not living together then just the fact alone that he was there visiting his daughter would make no difference on your claim at all. I think there a little bit more to the story than what you are saying. It doesn't add up that you claim has been deyaled for so long just for having a car parked at your house "sometimes".
Bit harsh what was said earlier "best of both worlds!! Of course she has to maintain contact with the childs father, that shouldn't be seen as a bad thing at all, or stop her from claiming her payments is she entitled to them. Maintenance alone isn't going to be enough to get by on. If you are recieving maintenance then you should declare that to the social welfare officer as well, or you might find they will just cancel your application and if you really are not together anymore, how are you going to support yourself and your daughter.
Come clean explain why you lied, if you have depression then you should of seen a doctor. Ask him/her for a letter saying that and the date you first went into him/her.