Thanks for the further replies. The system has never worked all that well, we have always had the problem of some degree of overspill and lack of control of the system. I neglected to mention that we have large Victorian style radiators which look great but do stay hot for quite some time after the heat has been switched off to them.
The point about over-run is interesting, that seems easy to find out and also the circulation speed. I’ll see what I can sort out this week.
If there have been problems from the start then gradual sludging and/or scaling is less clear as a cause of the problems but when did you last flush the system and add new inhibitor?
The pump speed setting will determine the flow/head point where it operates. CH pump speed is normally set, AIUI, to the minimum speed where all zones receive good flow. That seems to be one of your problems hence an issue to check out.
Then, I was looking for a potential cause of leakage other than bad valves. Since you write that they have already been replaced without impact on the problem.
A fault where the pump overruns for a short while post-burn as normal [1] but against a completely closed system will cause pressure to rise towards the maximum the pump will generate. Unless there's a working means to handle that (e.g. a bypass valve - but there are other means too), that might force flow past closed valves. I am not sure, however, if this is a real issue or not without knowing the system and testing it.
If you don't have the knowledge or means to test possibilities yourself, it's definitely worth asking your plumber if he/she knows any possible causes of your zone leakage problem other than poor zone valves. And the poor flow to your upper floor. My experience is limited but possibly enough to ask an expert a pertinent question in amongst nonsense.
And by "overspill" do you mean hot water flowing out of a vent pipe into a header tank? AKA pumping over (assuming the system is open-vented - not sealed). Pump-over might also be caused by excess pressure but also happens from bad plumbing - the cold flow, the vent pipe and the pump in the wrong juxtaposition. And if that's a problem you have, an installer might have set the pump speed lower than optimum to reduce pump-over.
If this were my system I would be checking, and hopefully eliminating, the above to try and get to the root causes.
[1] Commonly used to protect a boiler's heat exchanger from transient overheating when flow stops at the same time as firing stops.