Sold car but debt collection agency is after me for non payment of ticket from new owner

In Febuary last year I sold a vehicle to a friend's older brother. We completed the V5 document (I also have a picture of this) and this was sent off. The new owner (my friend's brother) then insured himself on the vehicle and also taxed it (as the law had just changed that tax can not be sold with the vehicle).

I never questioned if he had received the new log book it never even crossed my mind and didn't speak to him much anyway.

Today I've had a letter from a debt collection agency, on behalf of my local council, for a PNC which was issued after the sale of the vehicle. I've received no prior correspondence regarding the PNC (I assume it was a parking ticket stuck to his window) and only now been made aware. To add to the stress, the cost of the fine has risen due to no action being taken on it.

Where do I stand with this? I can't provide proof of sending the V5 apart from a picture I took of it completed prior to posting. However he was insured on the vehicle and will have completed the car tax documents etc, but I'm not sure if this will provide sufficient evidence to prove the sale.

I assume the response will be "You should have checked with the DVLA that the logbook form was received and processed", as I don't believe any correspondence is sent out to notify of the change, to the previous owner (I may be wrong on this).

Any advice is greatly appreciated.

Thanks.