return of bond

Tenancy Deposits

My tenants moved in dec 2005, during the corgi inspection of Nov 2006 the engineer condemned and disconnected my lovely coal effect gas fire as there were insufficient coals. The tenant stated her child had chewed up most of the coals and so she threw them. Now they are moving out and seem to have forgotten about the fire and expect ALL their bond back. I have been told that I cannot replace the coals for the fire as they need to be specific for that one and therefore the whole fire needs replacing. My question is...... Should I replace the fire and take this out of their bond........compronise and just take half the replacement cost .or just write it off? What would you do?