Sorry forever_missing, but Clairio whatever you do, DON'T ring the RSPCA!!! Phone CP if you need to, but not the other dreadful organisation.
It sounds as though your neighbours couldn't care less about these poor kits. My suggestion would be to wait until the REAL neighbour returns (if the woman you spoke to was telling the truth: she may have been the resident, but ducking the issue by saying she wasn't), then speak to them, making it clear that you would like to take the kittens in yourself. My guess is that they would allow you to have them.
On the other hand they may be giving you the "ownership runaround" because they fear reprisals (neglect etc). If this is the case, they wouldn't notice if you took the kits without saying a word.
Do you think she was telling you the truth? If not, just take the kittens in. If you can't afford to keep them, phone the Cat's Protection, offering to foster them until a permanent home can be found.
As for the pregnant mum: you could either allow her to have the kittens, or take the less pleasant path of having her spayed and aborted. I couldn't do the latter, but it may be more practical.
Good luck.