If you've found one, there will almost certainly be a few more ......
You can get spray from the vets, I used Indorex, another one is Acclaim, you vacate each room for a while and spray it over all furnishings, nooks, crannies etc, and it stops the eggs hatching for up to a year.
I'm not sure which flea treatment you used, but almost certainly the only ones which work are those from the vets, i.e. frontline, advantage, those ones.
However, don't treat Leo again with another dose until you've spoken to the vets, as overloading kitty with things like that can be dangerous x