phleb I have had a little think about this since last night and perhaps the best solution is for you to put your husband in the empty property and let the cats live with you?
Joking aside and to take some of the points you made in your last post, the RSPCA has protocols that all officers have to follow. One of these is not to take anyone's word but to check for themselves. Although you considered it "wasted everyones time" by them leaving a note at the house, this is a protocol they HAVE to follow. I'm sorry you considered it a waste of time because it wasn't. The situation has been assessed, the officers saw the cats were in no immediate danger, were being fed, thanks to you and your neighbours, and they are monitoring the situation. As they have no room, I don't see what more you expected them to do. Each RSPCA shelter has a licence to house so many animals, if they break the terms of that licence Environmental Health will be down on them like a ton of bricks if they get to hear about it. I know of one rescue charity in Leeds that had a licence to house 85 cats. After the kitten season, because of irresponsible idiots like your tenant not neutering their animals, they ended up with 215! Luckily nobody reported them but it gives you a glimpse of what an enormous problem this is.
Having said all that, your best bet with these kittens would be to say they have been 'abandoned' because that is a criminal offence and something? would be done. Unfortunately by you feeding them etc, they are now not abandoned. Hard as it is, and I don't think I could do it, the thing to do is to NOT feed them, then they will be abandoned again. The risk in that is that the boy could be PTS, the female and her unborn kittens will be OK. Because she is pregnant she won't be PTS as although she wouldn't suffer, the unborn kittens would, hence they won't do it.
An RSPCA officer I know, faced with putting the umpteenth unwanted healthy young cat down, after trying everywhere he could think of to find a place for him, drove to an area he knew where there were lots of old people's bungalows. He knew it would get looked after there and he was right.
Sorry for the long post, it has taken me ages to write it but I hope you now understand a little more.
Oh, one last thing phleb, I think it might be a good idea to you check on your tenants during their tenancy. Whenever I have let out my own property or indeed rented one myself, 3-monthly (I think) checks were always carried out. I can only assume that you didn't realise what horrors were going on in there.