Within a few days we had this.
The surveyor informed us it was plaster fungus it appears there was a small hole in the roofing felt and the water had been seeping in and it looks like the some of the roof timbers will have to be replaced as well as the roof.
Patrick managed to remove the fungus and all of the ceiling tiles and we have been waiting for the contractors to come and repair the roof before we can take down the ceiling and have it replaced.