Looks like lime mortar to me could be mortar or lime plaster I would leave in place and give it a few coats of Unibond or similar PVA mixture to bond it together then retile
Cheers, it's a late victorian house so that seems a distinct possibility. I'll give that a try. Any suggestions on how much to water Unibond down by, the internet appears to have wildly differing views.
Looks like lime mortar to me could be mortar or lime plaster I would leave in place and give it a few coats of Unibond or similar PVA mixture to bond it together then retile