Just to close this one out for anyone who comes across this thread in the future.
Repointing and lapping the felt over the back gutter did not resolve the issue.
After a long wait (9 months!) I finally got a long established and well regarded roofer to investigate.
The issue was as originally suspected... the chimney DPC was below the roof line so water absorbed by the chimney dripped off it into the loft. There was actually another DPC above the roofline (perhaps they realised they'd placed it too low originally), but this was poor being constructed of rolled DPC that did not cover the full depth of the bricks and was not a single piece so was effectively useless.
Repointing/refreshing the pointing, flaunching, existing lead may have prevented the issue for a time, but it would eventually have re-occurred as these measure deteriorated.
Resolution was to drop the chimney to roofline, insert a lead tray, rebuild the chimney and refresh all the leadwork.
Repointing and lapping the felt over the back gutter did not resolve the issue.
After a long wait (9 months!) I finally got a long established and well regarded roofer to investigate.
The issue was as originally suspected... the chimney DPC was below the roof line so water absorbed by the chimney dripped off it into the loft. There was actually another DPC above the roofline (perhaps they realised they'd placed it too low originally), but this was poor being constructed of rolled DPC that did not cover the full depth of the bricks and was not a single piece so was effectively useless.
Repointing/refreshing the pointing, flaunching, existing lead may have prevented the issue for a time, but it would eventually have re-occurred as these measure deteriorated.
Resolution was to drop the chimney to roofline, insert a lead tray, rebuild the chimney and refresh all the leadwork.
