We had a (chargeable) visit for our (non compliant) fosse a couple of years back. The SPANC man told me it would cost X (can't remember how much but I do recall thinking that it was flipping exorbitant) but...that if I agreed to upgrade the system then he wouldn't charge me.
So of course I did. Verbally. :)
We knew our fosse didn't comply with any regulation known to man when we bought the property in 2009 but I'm not planning to do anything until forced to. Which I should imagine, will take some time. In any case, the center of the village is now being re-vamped and mains drainage being installed. At the moment we are too far away but in the future, who knows?
Like Neil, I've heard mutterings of funding for upgrades and that's another reason to sit tight.
Mind you, I probably won't be saying that the next time the thing backs up!!