Hey all, following. Thanks.
So to answer a few points.. the cassette I took off operates a simple deadbolt that comes through the assembly, into a gap which is exposed when the handle is thrown up.
Beyond being the right size for the assembly and attached to it, it has no other interaction with it.
John, it does indeed lock and unlock "from the inside"(though not a key hole on the inside) and also locks easily from the outsid, it's just the unlocking from the outside. Which has gone wrong on 5 out of the 6 keys on the same day.