Well, to make a short story long, I checked all those components and traced the input jack up to the main multi-pin thick film but still to no avail, I could find no fault and was starting to worry I'd bought a basket case, if that thick film was duff then it's essentially scrap, especially as closer inspection of the board revealed it had been reworked at some point in the past.
Funny thing though, inspiration struck as I realised I was tracing from the positive terminal, when I checked from the negative terminal, it's all open circuit.
So, after a gruelling five minute fault finding session, I traced back the comon terminal (hindsight is a wondrous thing, see later) and found just after the current shunt something that looks like a chip inductor.
Unfortunately it measured ok, so I traced on from there but found I'd lost continuity from the common terminal, sure enough, the inductor was cracked.
For now I've soldered it back down as I can't identify the value, it's got no marking and my LCR meter won't measure it.
Seems to be working just fine now with my cheap Chinese AD584 reference, and if the pics will attach....