Worcester's been having a day with family
I think we're there, and having gone round in circles as we've gotten our heads around it a combination of MissPlumb's and Nostrum's responses answer it.
i.e assuming the heating circuits use a mixing valve (room stat, external temp sensor, weather comp) and are managed by the boilers inbuilt systems, then the buffer provides instant heat the boiler then fires up only working at the level need to maintain heat demand, it also monitors the buffer, so if there is also a hot water demand or if the buffer tank is cooling - part of the heating return has to go that way if flow is coming from it, the boiler will ramp up and then after heating demand ceases, it will continue to heat the buffer up until it's replenished.
Needs a good boiler controller, so won't work properly with some of the simpler boilers out there.
ETA show basically the same configuration, so it works when the boiler has a comprehensive control system - this schematic presumes using the inbuilt eta controls.
If you're NOT using the built in controls and / or the boiler has simplistic management, or a cheap buffer that doesn't stratify well/properly, then I can see possible problems with plumbing it the 2 port way: increased cycling, buffer tanks not heated properly, poor performance of the heating sytem (client perception).
So what do Windhager / HDG recommend? - I know that Okofen reckon that they don't need a buffer on smaller systems perhaps for similar reasons.
So:
Using Froling/ETA built in heating controls, using indoor and outdoor stats, mixing valves on the heating circuits and their weather comp system, the boilers should run more efficiently. Reducing the flow through the buffer also helps to maintain good stratification.
If you're not using their control systems then a 4 port buffer is a 'safer' configuration as far as ensuring heat is always available, though may trigger greater full power cycling as opposed to modulation; subject to: programming the correct buffer tank control, by using multiple stats - as a minimum top and bottom stats, and now looking at the hysteresis programming of those stats. Once again the "simpler" boilers may not be able to handle those types of inputs and modulate properly.
I think we've finally answered my original question!
For the simpler heat pumps we have developed our own buffer controllers specifically to address this using two stats and hysteresis management. - The controller is less than £100, needed a bit of lateral thinking to find it though
So next up is how do you configure the buffer tank control on / off points, what temperatures, what hysteresis?