I've been using quite a few rolls of fibreglass insulation (Knaul EcoRoll) that has a very similar look - It claims to be made from recycled glass which is probably the reason for colour.
Thank you for your replies. I have found some old insulation under the newer insulation, looks and feels similar but in smaller pieces. Does this look ok? Thank you for your help. I think the white stuff is old lime mortar from when the tiles were back filled.