Never mind what the seller told you, can he provide written evidence of the car's service history? If he can not, walk away from the deal unless you fancy some expensive repair bills.
Relying on a video and the vendor's patter to inform a purchase decision is something you will regret for ever. Go see the car, drive it, and examine the paperwork. If you don't feel confident enough in your own judgement take a knowledgeable friend or get the car independently examined. I believe the RAC or AA can provide that service.
Above all don't rush into a decision. If you miss out on this one there will always be another.