Help with mystery ceiling leak

[Mr.B-Campag]

I've got a leak in the ceiling above the bathroom which I can't figure out. Hive mind please help! I've attached a couple of pictures here. The ceiling has beams running across it that slope slightly downwards towards the centre of the house (no idea why) and the other day water started leaking from one of them at the lower end. There is a storage space above the bathroom but it's dry as far as I can see, certainly above the spot where the leak is there isn't any moisture and there doesn't appear to be a leak from the roof anywhere else in that space (which I can only look into, not fully access). The boiler is up there but on the other side away from this leak (and so not touching this beam). The beam doesn't go through the external wall and I don't see any signs of moisture ingress there either. Am I missing something obvious?

 

[classic33]

Condensation forming on the lowest point, which is under the heater?

 

[Ming the Merciless]

Is @Accy up there?

 

[Mr.B-Campag]

Condensation forming on the lowest point, which is under the heater?

It's not under the heater, and also coincides with the first rain for ages...

 

[Mr.B-Campag]

Am I assume what look like beams are, at least to come extent, hollow - as otherwise water could not flow through them? Not that it solves the mystery...