Downstairs underfloor heating is dead due to electrical problem

Hello all, I'm hoping someone can advice me on my heating problem - I seem to have lost all downstairs UFH, but the upstairs heating and hot water are fine.

About our system:
We have a gas boiler feeding an 5 zone UFH system downstairs, some radiators upstairs and the hot water cylinder. This is all controlled by an EPH timer which controls the water and a single heating zone (i.e. all the heating). The timer seems to be wired back to two different wiring centres - one (EPH model) seems to handle the upstairs heating and the hot water, and a second one (mystery manufacturer) which seems to cover the downstairs UFH. Into this wiring centre seems to be the cable to the timer, all the downstairs thermostat connections, connections to the actuators on the UFH manifold, and the UFH pump and valve.

What happened:
Today I went to replace a thermostat in a downstairs zone and I noticed that once I did this, it seemed to kill the power to all the downstairs thermostats (the little lights went off). The thermostat I was installing is a basic EPH CM2 model that had perviously been installed upstairs before I replaced it with an electronic 7 day timer model (which has worked perfectly for a few years now).

What have I tried/measured:
I immediately disconnected the new thermostat and checked the EPH timer which seems fine. It still seems to control the hot water and upstairs heating. I took it off the wall and checked the voltages on the back plate L & N are fine. I even briefly directly connected the L to terminal 4 'CH ON' - no difference (light on thermostat still dead).

Returning the timer into position, and ensuring it was set to 'heating on', I measured across the L & N on 2 of the downstairs thermostats - both are dead (previously it was 230v).

Possible conclusion:
I can't help but think that something has somehow blown in the second wiring centre. It's a mystery model - the only thing that looks like a model number is 'SAM 8.1/1' and some of the text looks to be German. It's more than a basic set of connector blocks, with 2 relays, some capacitors, a small traffo, a push-in-twisty knob (potentiometer?) in addition to all the connectors; but it has nothing that looks like a fuse.

If I knew who made it I could get try to some data on it and try to understand more about what it's doing. I'll try to post some pictures of this mystery wiring centre

So I'd be very grateful if anyone could advise, or even any tips on how to get more info on the wiring centre. Finally, is it a safe assumption that the EPH timer is not the problem given that the upstairs heating is ok, running from the timer's single heating zone?