Some good news with this... After wiring to exclude the pressure stat from the circuit, the boiler will heat up to a pressure of 0.5 Bar... which is a significant increase over 0 Bar. So the pressure stat is at least partially to blame.

Am I correct in that there is nothing else controlling the cycling of the heating coil though? What would have caused it to audibly stop at that pressure, rather than continuing to rise?

I can't remember every post but did you check all the electrical connections? What is the steam like at 0.5? Pretty lousy I guess. When it starts to heat up does the level change in the sight glass? I assume it's not firing up again after the pressure drops from 0.5 bar. Did you read the element resistance like erics suggested?

You're doing everything right but I'm stumped. I'm starting to think element. Where's Stefano?

Can you measure the resistance of the element?
Is it maybe a 220V unit being fed 120V?

Thanks so much for all the feedback, you guys are great.

I pulled the heating element out of the machine, just to check for anything overly suspicious. The first observation was a remarkable lack of scale buildup. It's pretty clean... But while the bottom half of the element was caked in your typical white scale, the top half was black... So I'm guessing someone let the water level in the boiler get too low, and baked the element a bit. Whether that would be sufficient to cause a loss of heating power like we're experiencing, I'm not sure.

Good news - it's indeed a 110v 800W element.
Bad News - being the nice guy i am, a friend borrowed my multimeter. Will measure the resistance across the element as soon as I'm able... I'd like to conclusively prove it's bad before replacing it.

Thanks again everyone!

Voila.

Heating element has a short. It's measuring resistance between one of the poles, and anywhere else on the machine... Resistance through the coil is 8K Ohms, if that helps anyone in the future.

wiring all checks out great, no problems there.

A sincere thanks for all the assistance, I would have been here for years without you fantastic people. Will hopefully report back with some fantastic espresso in hand.

Wanted to follow up regarding my prior difficulties. The trouble was indeed with the heating element. After removal from the boiler, it looked fairly obvious that it had been boiled down, or turned on without water as there was charred scale all over it, and a couple of places where the integrity of the coil looked highly questionable.

So, new heating element, and the thing works an absolute treat.

Conclusions that may prove helpful to others in the future:
If your machine isn't regulating the heater properly and it's not shutting itself off: It's probably a pressurestat that's stuck.
If your machine can be left on, warms up, but never gets anywhere nearly hot enough, the heating element may be compromised. Checking the resistance through the element as described above should clearly indicate if the element is cooked or not.

Again, thanks to all for the help...