It is certainly illegal to dispose of human waste in a bin under Environmental Protection Act 1990 which is hardly a new law having been in place 36 years . Nappies and incontinence products and the like in small quantities are permitted. Pointless complication having both a cassette and holding tank. As you have said a spare cassette is the easier solution.

Thanks, that's interesting and I hadn't realised that... 😊 So logically the authorities must care rather less about the safety of Cat D boats, otherwise they wouldn't allow the risk of such (unqualified) self-certification. Which logically should mean that all the other rules should be relaxed too, for example much-simplified RCR/BSS rules which only address the bigger genuine safety risks -- that would also cut down the time needed to do a BSS inspection, and presumably make it cheaper... 😊 I'm sure many canal boaters would be delighted if this was the case, but I somehow doubt it will happen... ☹️