Hello, my name is Ben and it is my pleasure to assist you with your question today. Was the seller a private or a business one?
You have rights to the extent that the seller appears to have acted in breach of contract by not honouring the agreement they had with you. As part of the sale it was specifically agreed that they would deal with any issues that came up once the servicing was completed. If they have reneged on that promise then technically they have acted in breach of contract and you can either try and cancel the sale or pursue them for the additional costs that may be incurred for making the necessary repairs.
The key is they are in breach of contract and in that sense you are entitled to be placed back into the position you would have been had the breach not occurred, either by cancelling the same or getting the repairs paid for.
You can contact them and make your position clear and state that you would either have to pursue them for the money and return the van or you would have to undertake the repairs yourself and then pursue them for the costs you have incurred as a result.
then you will have to decide whether to take the matte further. Your only option really is the small claims court, which is generally a straightforward matter and not too expensive
Please let me know if this has answered your original question or if you need me to clarify anything else for you in relation to this? Thanks