Get them to refund you £100 for every BHP missing.
If your not happy with the 92BHP to the point you do not want the van and the advert stated in writing that it was 112bhp (when new) and its actually the 92BHP model then technically they are in breach of the trade description act of 1968
http://www.legislation.gov.uk/ukpga/1968/29 and you are entitled to a refund.
If it states on the V5 that it is 92BHP then it may be difficult as it could be argued that this should have been checked at the time of purchase.
However as per your post the V5 states engine code SS, then this is not immediately obvious and I think you have a good case for a refund. If they know their legal rights too however you could end up going to small claims court, so you may have to go down the good will gesture route (£100 per BHP mis -advertised).
I reference chapter 29, section 3;
(1)A false trade description is a trade description which is false to a material degree.(2)A trade description which, though not false, is misleading, that is to say, likely to be taken for such an indication of any of the matters specified in section 2 of this Act as would be false to a material degree, shall be deemed to be a false trade description.(3)Anything which, though not a trade description, is likely to be taken for an indication of any of those matters and, as such an indication, would be false to a material degree, shall be deemed to be a false trade description.
In their defence (section 24), if they have relied on another source of information and they can prove it then then they will get of scot free, but im sure if they are a dealer then they would have this sort of information to hand on the computer so it would be lack of diligence.