Leaky roof woes: Season 1 Episode 2
"Hard to pin down roofer" turned up today and within 10 mins found the issue using a hose. It looks like a combination or poor valley edges that can't cope with so much water (plus were jammed with some old mortar and leaves that were pinned by the slates) combined with a very slight overhang on the new apex roof that is causing water in ingress under the slates on the OG roof.
Even though it should be the builder who did this bit of roofing to fix, I can't be arsed with the hassle, so hopefully we'll get the roofer to fix this week before the deluge of Scottish rain hits at the end of the week.
If anyone cares, looking at the below picture, its the overhang/detail in the top left of this photo plus a section of the valley a little further down that is the culprit.
What that doesn't explain is why there was wet/mouldy gyproc when the OG roof was opened up, but hopefully that was an older leak that has since been fixed.