1. Tear down existing boards and polystyrene and install Celotex/Kingspan
type insulation, then re-dryline and skim.

A further advantage of Option 1 is that you can install a reflective layer
between the tiles and the insulation. This vastly helps in the "hot in
summer" stakes. It will also result in a thinner result for the same
U-value. It is also possible to ensure that correct ventilation has been
installed. Someone may have stuffed the entire cavity, leading to rotting
problems in the rafters.

Christian.