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One question @Kaos asked was flow direction? We need to know which model boiler. Are the connection locations different on the two combi models?

On this Navian combi, from the installation manual, the boiler out S, and boiler return R, are on opposite sides of the case. Center two connections are DHW.

In the OPs installation pics it look like the connection for heating are on the left? If so which is boiler S and boiler R? Hard to see from that camera angle.

Ok. The flow is the right way and the plumbing is proper primary secondary loop. You don't need the boiler on closed space Ts for this setup, what you have there is good.

Assuming no air lock and pumps are OK, maybe the issue is the way over pumped setup. If you turn off all the zones and only run the one that has no heat, does it work? If this works but when you turn on a zone upstream of it, you loose all heat, the problem is the previous zone is flowing too much and using the full capacity of the boiler. If this is the case, the easy fix is to throttle the pumps with the isolation valve on the outlet of it.

If running the zone by itself does not generate heat (assuming primary loop is hot), the problem is air locked zone, gummed up piping or bad pump.

It's worth looking at how spot gas prices (and as a result electricity prices) have fluctuated over the last few years in Europe and the UK. Funny how supply and demand works…

ha that’s pretty dramatic - Nat Gas is the predominant heating and electricity source. We can use it.

Better off not using the checks installed in the circs.

What works in the lab doesn't always transfer to the field.

OK if there is 120 VAC across L1 and L2 and no 24 VAC, if you have not, I would also check the 5 Amp fuse to the left of the transformer (the big thing in the middle of the board). It is defined as a 'Pump Fuse' on the schematic, however it is not shown on the ladder diagram, so I have little confidence of its placement in the circuit.

I would double check all measurements.

Other than the fuse with the symptoms of 120 VAC across L1 and L2 and no 24 VAC on the other connector the most likely cause is the transformer failed, or a failed solder connection(s) on the circuit board could also cause this symptom. The board would have to be removed to inspect the solder joints for cracks also.

With the ohmmeter function of the multimeter and the system power OFF (circuit breaker OFF), I would verify the continuity of the primary and secondary of the transformer, some things may have to be disconnected and/or the board removed to get an accurate reading of the transformer.

It is possible to replace the transformer if a suitable replacement one can be found, also failed solder joints can be re-soldered. I suspect that whole board is proprietary to the boiler manufacture, so if replacement of the board is the best remedy for you, you may have to contact the manufacture or their parts distributor.

Just saying; If it was my boiler and it is a failed transformer and I could not find a suitable replacement of the circuit board mounted transformer. I would remove the failed transformer and connect up a generic 120 VAC to 24 VAC transformer and mount it elsewhere in a safe place. I can easily do that type of repair, also I realize others can't or are not comfortable doing so. I suspect a typical HVAC pro may not be comfortable with that type of repair, they would want to replace the board or the whole boiler if the board is not available any more.