As time progresses and seemingly ordinary cars now exceed £40k new, are car dealers legally obliged to advise the customer of the extra £320 tax per annum on any second hand car before they commit to buy, and should they state it upfront on any advert?

There's probably no legal obligation, just as there's no obligation to show the high VED on a 2007-17 gas-guzzler - like the insurance group/cost, it's something for the buyer to check.