Daniel came round at short notice. He took one look at the job I wanted doing - plastering four walls in a new outbuilding - and advised me that the material used was not plasterboard, and was therefore unsuitable for plastering (as the plaster would crack). I have since verified that it is indeed gypsum fibreboard.
At my request he advised that hypothetically, if it were to be replaced with plasterboard, the cost of the job would be 450 GBP. This is to the penny what I was expecting to pay, so happy with that.
**Disclaimer - no work was done so cannot comment on quality of plastering, only service and tradesmanship.**