Is it 'fair' to say it's not my responsibility?
As you are in Europe, I recommand that you check the European Vehicle Inspection, its autofore link, and related links on the right menu. Give great advice and help with the laws.
In Europe, all vehicles must have a technical inspection (new car: after 4 years, then, used cars, every 2 years). In case you sell a used car, as you did, the test must be done by a certified inspection center, and be less than 6 months old.
You can't sell a car without this inspection:
- If you did not have it done, you're in trouble.
- If you did it, you're fine. Everything that comes after is not your responsibility any more.
Can I say I don't wish to contact her about the car?
Now, you're left with the moral part of the deal. I sold some used cars over the years. Good ones, with the inspection test done. No worries. But I would just answer her, listen to what she says, and if she wants to blame you for anything that happened after the deal, politely decline.
If you don't answer her, she might want to get you into trouble, or go to the police/court, and it will only escalate. And cost a lot of money. I would definitely listen to her, so I know what she wants, then, it depends on what she says/asks...
EDIT: after you (OP) mentioned that according to my legal advisor I am not liable for the new damages, and that you would like advice on how to politely hold her off, I'd say a ferm and polite way would be to let her know that you won't go any further into arguing over this issue:
I am sorry to hear from you that you had some problems with the engine. We had the APK done, and it was OK. I gave you the proofs/bills for inspection and repair. We then agreed and had a legal deal and sale. Unfortunately, there's nothing I can do about that anymore, and I am neither responsible nor liable for anything that could occur after the sale. And I can't be of any kind of help, as it is your car now, and I can't legally interfere.
I hope you find a solution on your own, and can get the engine fixed.
And nice closing greetings with your own words...