Welcome to world of V50 parking brakes. As Clan has already stated, there was a modification which included an insertion in the handbook about 18 months ago. It doesn't really address the weakness in the design, but just explains how to overcome it. Frankly my wife still can't apply the brake so that the car holds on our steep drive. Basically if the car (and brakes) are hot when the parking brake is applied, then when evrthing cools and contracts, then the brake may not hold the car. Set the brake pressure with footbrake and then hold with the handbrake. Just be aware and learn to live with it and enjoy the car. By the way my, old V70 would roll fractionally backwards when the parking brake was applied on a hill, before the brake grabbed the disc and locked the car. I got used to that too. The car would always hold though, no matter how hot or cold.