The insurance company says the vehicle has been repaired properly, and as a result, not suffered Diminished Value. Is this true?

No. The vehicle is still worth less regardless of it being repaired properly due to the accident. The fact is, you cannot sell it for the same amount as a like vehicle which has not been in an accident. And, no dealership will take it in as a trade in for the same amount. Insurance companies have tried to avoid paying on Diminished Value claims for years and they do and say whatever will justify their position. Diminished Value case law has been around for nearly 100 years. We can collect from the at fault party's insurance company!