However, I misread the V5C2/3/4 slip and forgot to send it off to DVLA

As the purchaser you do not need to send any of those slips to the DVLA. The V5C/3 should have been sent back by the previous owner when they sold the car, the V5C/2 should ahve been kept by you and not returned to the DVLA and the V5C/4 is only for when a car is scrapped or exported so again, nothing for you to do.

The new V5C is generated when the seller sends the old V5C back to the DVLA with the new keepers details on it in section 6 and the document is signed in section 8 by the seller and the purchaser.

Cannot see how you have done anything wrong really but failing to notify them of a change of address may count against you.

Example

Last December we bought a Fabia and the dealer sent the V5C back after we had both signed section 8. He gave us section 10 (V5C/2) to enable us to tax the car which we did using the salesman's PC.

For our PX he kept the whole of the V5C except section 9 which we both signed and said he would forward for us. I declined his kind offer and sent it myself after copying it, I wanted to be 100% sure it had been sent on the lead up to Christmas.

The new V5C dropped on our doorstep about 2 weeks later.

I am totally confuesed why they gave you the V5C/3, as above that should have been sent to the DVLA when the dealer acquired the car (preferably by the previous owner).

And equally confused how you have been taxing the car since that first time. The V5C/2 sand its reference number should only be able to be used once.

or if you might have any advise yourself?

It needs to be done now.

With regards to notifying the DVLA of the change of ownership that is the responsibility of the dealer or in the case of a private sale the registered keeper (i.e. the seller), both have to send in the front page of the V5C (sections 1 to 8) to the DVLA filled in and signed by both the seller and buyer. There is a fine for not doing that but its the dealer/seller that gets that fine.

With regards to changing address that is your responsibility and again there is a fine for not doing it. According to .gov its up to £1000.

I can only presume the V5C must have been lost in the post either on its way to Swansea, on its way to you or even disappeared into a black hole within Swansea.

Your problem is the time since you bought the car and subsequently moved house. Will they believe you when its so long?