Old plaster blown

[rreanor]

I have a patch of blown plaster (in a bathroom I'm renovating) over what looks like a cement based scratch coat (which feels pretty solid). Should I remove the remaining blown plaster and patch this up with new lime undercoat? Ill be tiling over this as it's right next to the bath.

This is a double skin Victorian building without a cavity. It's not a damp building.

See pic.

Cheers