After working at heathrow for 2 years and getting to know the habits of the baggage handling staff, i can only say NEVER put anything valuable or fragile in your baggage. The bags are opened and searched routinely by staff who have had minimal checks done on them, are on a low wage and have minimum consideration for the contents and owners of the baggage they are handling.
Although we have not spotted anything missing yet, the cases have definately been tampered with and the contents were obviously just thrown back in.