What does Category D or Cat D mean?
It means the car has been written off in an accident.
Category D is the least serious of the five registered levels of damage. The groups range from A to F, A being the worst, suggesting that the vehicle is so damaged that it cannot even be used for salvage and should be crushed.
Groups B and C mean that the vehicle has been heavily damaged and the insurance company has chosen not to go ahead with the repairs, although group C is usually salvageable if the repairs are carried out correctly.
Group F means that the vehicle has suffered fire damage. Group D usually means that the vehicle has suffered light damage but whether the insurance company will repair it or not is dependent on the cost of the repairs and the value of the vehicle.
Until the vehicle has been inspected there is no guarantee that it has not suffered chassis damage. Your best option is to invest in a full mechanical inspection at an approved dealership, or by RAC Inspections.
Also, make sure you alert your insurance company to the fact that the car has previously been damaged before you insure it. They may not pay up for a claim if they are unaware of this.