Thanks guys, I think I got the Stronghold on, she is long-haired but pretty docile, so it wasn't as if we had to try and get a moving target! She spends most of her time indoors so I don't know where she might be getting them from.
Reminds me of my friend - they now have 3 cats, were 4 until late last year. One of them always seems to have the odd flea, no matter what they do, whilst the others are clear. At least their flea-bag is out a fair amount and they back directly onto healthland, so it is easy to imagine that she is picking them up from a hedgie or something.