Radiators hot even when only HW is calling - confusing

[tpot5]

I'm really confused about what is going on here.
The symptom seems simple: hot radiators even when CH is off but HW is calling.
But this one is confusing.

I have a Y-plan system...

Boiler: Potterton Flamingo 50s (pretty old but serviced).

Pump: Katsu circulating pump (model 151711).
Mounted vertically.
On 'flow' side with flow upwards towards the diverter valve.
On lowest setting of 45W

Diverter valve: 22mm 3 port ball valve. A, A/B, B.

Diverter Actuator: Corgi A3MID-C

Controller: Horstmann Centaurplus C27 series 2

Room thermostat: Honeywell, manual, circular wheel

Standard hot water tank.

Just to confirm a few things:

The arrow on the pump is pointing upwards toward the diverter valve.
The room thermostat is working (checked the CH calling terminal with neon screwdriver)
The controller is working (checked the CH calling terminal with neon screwdriver, and the other terminals too)
In an attempt to fix this I have just replaced the diverter valve and actuator.

The problem still persists.
So I wired up the old actuator, attached to the old ball valve, in parallel with the live one.
I did this so that I could see what the new one was doing (Big assumption is that I replaced a working actuator and valve when I needn't have).

So when HW is calling and CH is not, the parallel valve is firmly closing off the CH port.
But my radiator are still hot!

I hope someone can help me.

there are two types of valve.

IIRC the "mid position" returns to "both open"

but the more common 3-port returns to "HW on" with a spring, when at rest. You can hear it whir back into position.

I wonder if your system has a mismatch in motor head, or if the wiring was designed for one but now has the other.

I can't remember this clearly enough to know how to diagnose.

@Omg8
I'm focusing on tracing the pipework now, that's why I'm interested in the boiler.
I'm also now convinced that the valve nor actuator (which is brand new) is not the culprit following my manual valve test.

@PigletJohn
The manual valve test you suggested also resulted in the same fault showing up.

Update:

When the radiators run normally they all have forward circulation.

When the radiators are unexpectedly hot (HW calling, CH not) the original house radiators have reverse circulation and the extension radiators remain cold

Sorry just read that the valve and actuator have been replaced. Has this problem always been there or has it started recently?

Sorry for the long elapsed time for this concluding post.

The fundamental problem was with the way the upstairs radiators were originally plumbed in.
The upstairs radiator return was plumbed in to the hot water return instead of joining the downstairs radiator return.
I guess this was originally done for convenience and with a lack of understanding. It saved running an additional pipe between upstairs and downstairs.

This allowed the hot water return to run slowly around the upstairs radiator circuit in reverse.

The solution was to install a one-way valve on both the upstairs and downstairs radiator returns.

Thanks everyone for your help and interest.