I've just spoken with Rosie who said they've been offered ~20 lengths of 4"x2" from someone else who uses another hut.
She told me that the owners of this hut have lined their walls with 19mm ply. This seems OTT to me, as I reckon Steve's right that 9mm will do just a good a job at sealing in the crumbly wall panels, as well as providing a sound base on which to fix to.