We're interested in buying a house that has a fosa septica.

That in itself isn't a problem, but the owner tells us that in the 40 years that the house has existed, they have never had it emptied.
She said that early on they took a look inside and found that it was almost empty and that never seems to have changed.

That must mean that it's leaking away into the ground, which I imagine isn't legal.

Does anyone with more experience of buying property in Spain have advice on the best way to manage this?

That is perfectly fine - it's how they work. "Grey water" naturally seeps into the surrounding ground.

Some town halls now insiste that properties being sold MUST have a 3 or 4 chamber system so best make sure of this before putting in an offer.

Septic tanks are perfectly legal in Spain and, in theory, should rarely, if ever, need emptying.

I agree with Snikpoh.
I lived with a septic tank for four years and had no bother at all. It didn't need emptying in that time, although I did in my first year.

I know that in Valencia province it is no longer possible to obtain a certificate of first occupation with a septic tank and many who wish to sell are now installing the multi chamber tanks.
This is something that you must bear in mind.

I put in a multi chamber tank about 12 years ago. Never needed to empty it. The end water just seeps away bit by bit into my land. That's how they are designed.
I am wondering though if the OP has one like that or just the old one that does need to be emptied at some stage.

I inferred from the 40 years comment that it was the original fosa septic, pozo negro.

Certainly mine didn't need emptying and my Spanish neighbour made good use of his by siphoning the contents onto his vegetable garden.