Shared Cells

I have a few elements on my drawing (given to me by a third party) which appear as shared cells that I cannot do anything with apart from copy them. At first I thought they were just multi-line text but when I tried to edit them nothing happened, when I hover over them a box appears which reads:-

Element not valid for tool

Shared Cell: *U92 \ Text: dp

Level: AJ Drainage

There is no cell library attached with this cell in. The level is not locked or frozen, I have tried to Unlock, Ungroup and Drop the element but nothing happens.

When I open the Element Selection Tool and choose Shared Cell and then remove all other layers apart from the AJ Drainage apparently there are 150 elements. If I open the properties of the AJ Drainage Level in Level Manager, the Usage Tab only counts 2 Shared Cells.

I'm very confused, has anyone any idea how I can even just delete these elements?