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I have read that in timber moisture readings "over 12% are a cause for concern and above 18% timber is vulnerable to decay by wood rotting fungus"

I can find no information for paster / plasterboard.

We have had water leaking into a flat roof and need to know how much damage has ben done to the ceiling and how much work needs to be done to put it right.

Firstly dont worry yourself over how much damp is in the plaster, it make no difference. An electric condunctivity moisture meter cannot give you a quantitive reading for plaster. They are calibrated for use with timber. This is why you cannot find any information regarding readings in plaster.

The timber structure in the flat roof on the other hand is different. You need to know whats going on here. If the flat roof is leaking, then the timbers are likley to be at risk from rot and wood boring beetle attack.

First step is to get a roofer to fix the roof.

Second step remove the ceiling and allow timbers to dry.

Third step, repair the ceiling.

If you dont remove the ceiling then the timbers may stay damp and therefore remain at risk from rot etc..