Hi Steve
I'm not a criminal expert or even a consumer law expert, but i have had plenty of dealings with car manufacturers in my time with various issues and interesting outcomes!
My first suggestion would be to email/write to the dealer where you bought the van from. You need to give them a written report of the problem and wait for their reply. You now have a written chain of events.
Clearly they have failed to take you seriously and that would majorly annoy me. Your next step (IMHO) is to complain to the service manager at said garage and if necessary escalate that to senior management or if they are part of a chain speak to a manager/director of them.
If you still don't get anywhere then go to the manufacturers/distributors in the UK. I would have thought by then that someone would have listened and have taken you seriously.....
I took on my local Audi dealership in September- long and short i got to director level of the chain of owners of the local garage, resulting in a half price service and going to another local(ish) garage in the same chain,
I also took on a Renault dealership a few years ago regarding a gearbox issue on a 3 1/2 year old car -that resulted in going all the way to the top and spoke to Renault UK and pushed my way to a senior manager in their complaints department - i got them up to 70% contribution for a new gearbox, before deciding that my best option would be to dump the car.
Best of luck with getting your issue sorted.
Feel free to PM me for any further tips/advice or even bounce a few ideas around 