I think the mystery of the hull lining material has been solved. Suggestions have included sprayed cork and glue, but Simon reports that it is very hard and the small piece he was able to prise away does look like stone/cement.
To sum up - Portland Cement became popular in the mid-1800s. Many builders of iron vessels in the late 19th or early 20th century poured cement in the bilges as a protective coating. Compared to the best paint available at the time, cement served far better in protecting hulls from the corrosive effects of sloshing bilge water. I found all this on the Internet - yes my broadband is back again after 6 weeks!!
The clinching item came from the 1880 Specification for Irish Lightships, which stated “The inside of the vessel, as high as the sister keelsons, to be coated with best Portland cement; to be flush with the limber holes, and to be laid thickly in the throat of the frames, and to cover the nuts of the bolts."
I cannot fathom what ‘flush with the limber holes’ means, let alone ‘in the throat of the frames’, although I do know that a limber hole, 3 inches in diameter, was cut on each side of the keelson and a galvanized chain, 3/8 inch diameter, was passed through each limber hole for the entire length of the ship. These limber holes allowed bilge water to drain to the centre of the ship where it was pumped out from time to time and the chain kept the holes free of detritus (Photo 1231).
I hope I have got the diagram right – perhaps you naval chaps would put me right if necessary!