Found!

Found in the excavations in the bathhouse, a 19thC fishing tackle box. Various other bottles, masonry and a small almost child's size anchor! They can now feel solid floor under their feet and are hoping to expose it by the end of the week. The hope is that it will be the Reigate stone flagstones that Edward 1 bought for the bathhouse.
I am hoping to be able to get in there myself later in the week. Exciting stuff.