I spoke to the water board this morning and they confirmed the feed outside our front door serves us, and the flat above.
The pipe you see in the corner of that room is going up to the flat above, the stop tap is seized open btw.
The meter outside is spinning consistently, I asked upstairs flat to not use any water for 30 minutes, we stopped using ours also, but the meter kept spinning at the same rate.
I think this confirms there is a leak just under that wall (showing all that water damage).
What next do you suggest please; remove a skirting board, see if there is anything, then start breaking the concrete floor out to find the leak?
Also, it seems like the leg that serves the upstairs flat is the section that is leaking (under our floor), but I guess it's still our problem?
Thank you in advance for any help.