Your previous owner's contribution appears to be to create a high
risk of condensation in your roof by blocking the ventilation path
between the insulation and the roof felt.

Yes, I have had concerns about this, but the surveyor did not flag this
up [1]; and there is no felt, just original welsh slate laid on battens,
so perhaps there is enough air movement to keep things dry?

[1] I know, one shouldn't expect 750 pounds worth of surveyor's report
to tell you anything useful.