The V5C must be filled in and signed by both the Seller and new Keeper in section 8. This part of the document must be sent by the Seller to the DVLA immediately. Assuming that you are not a registered trader, Section 9 can be destroyed and section 10 ONLY should be handed to you as the new Keeper. As the vehicle was not being exported, section 11 can be destroyed.
You were under no obligation to send the V5C to the DVLA when you found it, but rather you should have returned it to the Seller for them to do so. Due to the circumstance of your now having activated a static speed camera, you really should inform the Seller of the delay.
The Seller is the one who is liable to a penalty and or summons for the failure to notify. As the requirement is to immediately send it to DVLA, it cannot be prosecuted after 6 months due to it being 'time barred' as a summary offence so the Seller would thus have a defence.