I didn't have any trouble when I bought the Seat he just took a photocopy of my ID and that was it. This seller is reluctant and hopefully with a little help a deal will be done.
I am sure there are a few and Nigel must have at least one I would of thought
The difference with your previous purchase could be the registration as a historic vehicle. Sadly this one is not, and the process to register it as a historic takes time and it is about 500 Euro, so it does not make sense for this deal.
As a standard registration vehicle, and in my knowledge (so take it as a provisional and non qualified advise), I can outline the following, pending confirmation with the Transport Department tomorrow:
1. The vehicle needs to be SORN'd. This is a simple process costing around 9 Euro for cars up to 15 years old and being free for older ones, that is your case. So zero Euros for this. This needs to be done by the seller or a representative at the Transport Department. He needs to fill a simple form declaring that this SORN is due to "export", or to be more precise while the UK is still in the EU, due to "Intra-European Transit".
2. You sign the contract, fill a couple of forms, and the car is yours. This can cost about 200 Euros or so if you involve a local agent. Then you have HMRC Equivalent in Spain, that will calculate the value of the car and charge some taxes. Who pays those taxes is a matter of agreement between you and the seller (in most occasions, a matter of disagreement)
From this point you have 3 options, I mean legally
a) You put the car in a transporter to get to the Ferry in Bilbao, and when you arrive to the UK you use another transporter to get home. In your case the seller has offered himself to transport the car to the port.
You put the car in a transporter to get to the Ferry in Bilbao, and on arrival you store the car at the port, waiting for UK registration.
c) You ask for a provisional permit and green plates before you leave Spain. That would allow you to temporary insure the car and legally drive it in Spain and all over Europe for 60 days, till you sort out UK paperwork. I have done this several times between different European countries in the past.
I don't know if there is other option on arrival to the UK with a SORN car, maybe some other autoshiters would know other ways around.
Well, option C sounds good for most of us, but there are a couple of caveats:
1. The car needs to be MOT'd in Spain. In your case, if the condition of the car is as expected, it would not be a problem (fingers crossed) Anyway it should be sorted in advance and would cost you 50 Euros. Only thing is convincing the seller to take the car to the MOT station.
2. You, as a person, need to have the infamous NIE (That is a registration number for foreign people either living in Spain or financially dealing within Spain). Good news is that it is not as difficult to get as people usually think (Well maybe I am being a bit over optimistic when we talk about the authorities of my country of origin) Let's say that it would take 3 to 5 working days to get it done at the Spanish Consulate in London, or you could do it at any Police Station in Spain (Which I don't recommend at all, delays are mental for anything ID related) More good news is that I can deal with that for you at the General Consulate in London.
Tomorrow I will make that call to the Transport Department and will update.
Hope this information helps anyone interested in the Spanish market.