Hi Sue,
We bought a van from a chap who had bought it on HP and then found his teenagers didn't like caravanning and he didn't see the point in struggling with the repayments. Not only did we get a nearly new van, we part exchanged it two years later for what we paid for it.
The seller was very up front, showed us all the paperwork etc. for the HP, he contacted Black Horse I think it was, who in turn spoke to us and we paid the money to Black Horse and the remainder to the seller. No problem whatsoever.
If the seller really wants to sell he will agree to this and it would be a shame to be put off if you've found a van you like.
I'd put the option of you paying off the HP to him and if he doesn't want to have an honest and open transaction I would think twice.