Up to you, but if you have the reg to hand and are happy to post it, I can possibly tell you why it was written off.
Cat D is the lowest of the total loss categories, and a vehicle can be placed under this for a variety of reasons.
The interpretation of Cat D is that the vehicle repairs are looking to equal or exceed the vehicle value at the time of the incident - dependent on the age/value of the vehicle, this can range from thousands of pounds to a headlight - I assume yours falls into the thousands of pounds area.
Equally there could have been no visible damage (ie flood causing engine failure).
In real terms though, the value of the vehicle would be affected, and in terms of any future insurance payout, you'd be looking at trade value (from Glass's Guide).
In regards to telling future purchasers, you do have an obligation to tell them, but obviously the person you bought the car off didn't tell you (assuming that they were aware).