On Tuesday, 14 May 2019 15:47:13 UTC+1, George Miles wrote:

anyhow with this week of hot weather it just needs lots of draughts and it will dry out.

Forgot to comment on this; everything feels bone dry. You would of course expect this with the current weather but there's no sign that anything has ever been damp other than the black marks on the felt.

One thing I haven't done is tried to look on the tile side of the felt - it could well be there's something on there that is causing the marks on the inside. I don't know what, if any, relevance this has but we're in a rural location any there must be a lot of pollen etc floating around which must get trapped on this teabag-like felt. In the past I've found the odd dead fly in the loft dangling from the felt with appears to be a leg stuck!