Bought 2nd Hand car how to tax it?

Hi,

I have just bought a 2nd hand car from a used car dealer. I am collecting it Friday. I know there are new rules on road tax. The guy at the dealer said he will tax it for me and then just tack the cost onto the price of the vehicle but I thought I had to do that after I bought the vehicle? Wouldn't he need my details on the forms? How would I know that he didn't later request a refund and then I was driving an untaxed vehicle? I can't tax it at the moment as I don't have the logbook or anything.

I'm a bit confused. Any help would be appreciated. My problem is also that I don't think the dealer chap knows what he is doing either as he is 'covering' while his Dad is on holiday....

TIA

Olly