Even though you're out of the 30 days you're still covered under the consumer law. If a fault occurs with a car or item in the first 6 months of purchase it is assumed the fault was there from when it was purchased.
My brother is going through this now and getting his car fixed free of charge. It needed an abs pump (£3k bill!).
Unfortunately you'll find you'll need the heated seats Wiring harness. Not only that you might find the airbag connector might be different. And you can't just cut off the old connector and solder on a different one as this will throw the airbag warning lights. As such the best way is to remove the airbag from your current seats and put them in the new seats if that makes sense.
Have the engine mounts and gearbox mounts been checked? As the movement could cause noise.
Could it be something to do with liquid sloshing when I get loads and it throwing off the sensors?
For example when I had an issue with my A6 washer bottle leaking it wouldn't always tell me it was low unless the washer fluid moved away from the sensor if that makes sense.
Seen loads of these stories. If selling a car make sure you're not alone. Make sure they either come alone or with one other person. Anymore people refuse to see them.