Does a car Dealer have a legal responsibility

I have an ongoing dispute with DVLA because they say they never received a V5C/3 (section 9).
DVLA say that the Main dealer who took the car in part ex should also have notified them and therefore they (the dealer) are at fault
I have challenged this statement with DVLA after several garages and main dealers tell me this is not true and that they don't, in such cases notify DVLA.
This goes back to May 2015, before DVLA launched on line systems for both parties to inform them.
I know the Dealer on line notification system is voluntary at the moment so the question is.

Has it every been compulsory or a legal obligation for a dealer to inform DVLA they have taken a car in part exchange?

The car in question was sold on by the dealer within 4 weeks when DVLA were notified of the new owner
I need to know because I am in dispute with DVLA and maybe should be in dispute with the dealer.
Any advice would be appreciated, thanks in advance