Bookworm, whilst I disagree with you about which is the top level document, I do agree with you that the POH is the definitive working level document. It does seem more than mildly wrong that the POH doesn't contain a clear list of minimum serviceable equipment since that is the document any operator has most available to them.
I took a look at a Cessna TCDS and that's no more clear, and from memory (I'm allergic to single engined Cessnas so don't keep their documentation around) the Cessna POH doesn't have a clear instrumentation MEL either. Neither can I find anything in CAP455 which is the other obvious place to look. Looking deeper, into JAR-VLA 1-App H which is a JAA approved sample format for an operators manual although it says there must be systems description, there's no requirement listed to state what should be fitted and serviceable for flight.
It's starting to strike me that although the information is there if you look hard enough, there is a definite weakness in current practice - this information should be out in the open. Anybody feel brave and fancy flagging this up to the powers that be?
G