Phone the council and ask for pest control.
When we had this problem, they sent a man with a backpack spray who did the job for £30. Very efficient. No doubt the service will vary in different areas.
They do it for free for people on certain benefits.
It might be that the Frontline didn't work, there have been a lot of reports of fleas becoming resistant to it. You might need to treat the cat again but I would talk to your vet first.
Fleas seem endemic at the moment, but Frontline's getting poor press at the moment. Switch to Advocate or Stronghold - but you'll need to clean up the house first, the eggs will now be lying in dark corners waiting to hatch.