Sorry, I'm new to the forum and not well versed in roofing lexicon. I've attached photos as you can see. Mine is the slate roof and the neighbours' is the other one with ceramic tiles. It is indeed a bonding gutter between the two roofs.
We live in an Edwardian mid terrace and have a leak in our upper front bedroom adjacent to the chimney stack we share with our neighbours.
Despite two separate roofers replacing the lead flashing around the stack, repointing the stack fully and re-doing the channel between ours and the...