Thanks for the input all
99% sure it's not air con or the heater matrix (been around those loops a few times) - but certainly worth a mention.
It has gotten a bit worse - which in a way is good because we can see drips now which might help track them down ->
https://imgur.com/gallery/qsriDAN
I took it to a smash repair place last week and they reckon there's a good chance it's the windscreen. The windscreen did leak from new and it took ford 2 goes to sort it out, so I wouldn't be surprised if they bodged something way back then and never really got it right.
Sounds like a sensible path is to remove some trim around the A pillar and see if we can notice any source now that it has gotten worse, then maybe look at pulling the windscreen, inspecting and re-fitting.