"I have been using some home made Pyrocat-HD, made of chemicals from Vanbar in Melbourne (Australia) who are generally pretty good with their source of bulk chemicals. Results have been promising, but shadow detail has been unexpectedly low. So I've done a proper Zone test on 120 HP5+ (I know I should have done it in the first place) and have found speed to be under 100. Contrast in Zones 3 and beyond is quite high (not a worry at the moment). I'm using 1+1+100. Non-rotary tank."
Something is very wrong here. Pyrocat-HD give full emulsion speed with HP5+ so your effective film speed is off by about two full stops.
"I suspected that the phenidone might have been stale (it was a bit brown) but the chemical guy at Vanbar said that some POTA had recently been made with that batch and it had been fine. This seems a good enough test to me."
Probably not the cause of the very low EI. Even without the phenidone the pyrocatechin in the formula has enough punch to get close to emulsion speed.
"I am pretty careful with measurements, but it's not beyond possibility that I made a mistake. Would adding too much potassium bromide be a cause of depressed shadows combined with minimal effect at higher zones? I intend to mix another batch very soon."
Adding too much potassium bromide could have a huge impact in depressing shadow density, and effective film speed.
"I've been a PMK user for a while and I've enjoyed the long shelf life of mixed PMK. The phenidone in Pyrocat-HD does give it a shorter shelf life. Also, I understand that phenidone usually requires a restrainer to control fog."
Pyrocat-HD also has a very long shelf life, definitely more than six months. Phenidone does require a restrainer because it has enormous regenerative qualities. Just a tiny bit of it, as in the PyrocaHD formula, results in a significant increase in efffective film speed.
"If I use metol (25g/Litre of Part_A) instead of phenidone could I dispense with the bromide altogether? If using metol costs me 1/3 of a stop I don't mind. I don't do rotary processing, and don't get much general fog from PMK, although the negs don't look as clean as the Pyrocat-HD ones. Would doing this affect the qualities of Pyrocat-HD when used with minimal agitation?"
You could substitute metol for phenidone at about 10X the amount, but I would still recommend that you include potassium bormide. The main difference with the metol substitution is slightly reduced film speed.