I would definatley contact the council or the RSPCA about the cat over population, if the cats have that many fleas they must be very uncomfortable and that is unfair for them.
The council would also probably like to know if all her cats are registered, and it costs more to register an un-desexed pet so that might urge the next door neighbours to desex their cats.
I know how irritating fleas are and they are horrible to get rid of, i hope that your situation improves soon and your next door neighbours pick up their act!
Goodluck