Ah, mine was a diesel. I can't remember encountering a 2L petrol.
Right: you can SORN the van: tell the DVLA that it's off the road and not being driven. You should get a refund. Speak to your insurers about the insurance and if you can't get out of it (which legally I don't think you can) then switch to third party and put up the excess to get the insurance as cheaply as possible. If the van is stored at the dealer's garage then any theft or damage occurring will be covered by his business insurance.
Go on the Autosleeper owners sites (there are several) and ask for advice and help. There are lots of enthusiasts and they know everything.
This one seems to offer specialist technical advice:
https://www.auto-sleepers.com/owners/asoc
Unfortunately I think you paid much too much for the vehicle in the first place. My Clubman cost me £15K 15 years ago. We loved it and used it regularly but decided in 2021 to let her go. 42k on the clock, running nicely, no major issues — and I was happy to accept £9k for her. I would suggest making yourself such a pain in the arse at the dealer's that they give you your money back to make you go away. They will probably have offered the previous owners £5, perhaps less for her in a part exchange so they haven't got a massive amount to lose.
Are there any technicalities you can catch them out on? Did they make any promises that they didn't keep to, which you can use to argue they broke the contract? When I bought our van the dealer promised to replace one of the tyres and the rear disc brakes, which were cracked. When I went to collect it none of that work had been done but he still expected me to drive it away. This was a big dealer, not some guy working from his front drive.
Given the length of time this situation could go on for, I might find £200 for a solicitor's advice. There may be selling regulations or the requirement for a three-month guarantee. I've known people in the past buy stunning Japanese imported vans and then have them off the road for months and months while spares are sourced in Japan but I've never heard of issues around VWs before.
I'm sorry and I wish I could offer more help. I hate hearing of Clubmans in distress.