After dithering for days, we have decided to remove the cladding from the ceiling. Let's just remove the stuff that has to be removed for the stairs and see. And it's rotten. Full of woodworm. Fortunately doesn't seem to have affected the beams or the flooring above.
As I got to the furthest corner, a bee joined me, and then another and another and another. Yikes. They are not honey bees unfortunately, a French friend said that they were bourdons but that translates as bumble bee and these are just half sized bumble bees.
My hero, Him Outdoors, very bravely, did a 'pull plank off and run' job and discovered this in the corner!
OMG!! Turns out that this is dead and no longer used but it doesn't stop dozens of bees from going in and out of the wall right in the corner. So we have a bee's nest in the wall. Hmmmmm more thought required...............................................