mine appears to have been solved!
looks like the fascia sat a tiny bit higher than the soffit on the old extension and the wind had assisted rainwater to get in between the joints, and it had then worked its way round to the wall, running down between the wall and the box gutter
a bit of 9mm cover board over the old fascia and lots of attention to the existing leadwork, with some new lead mate seems to have solved the problem
hope i haven't spoken too soon though...
looks like the fascia sat a tiny bit higher than the soffit on the old extension and the wind had assisted rainwater to get in between the joints, and it had then worked its way round to the wall, running down between the wall and the box gutter
a bit of 9mm cover board over the old fascia and lots of attention to the existing leadwork, with some new lead mate seems to have solved the problem
hope i haven't spoken too soon though...
