Where can i find the cheapest ferry deals for dover to calais mid sept, is it worth just driving to the dock and picking up a cancellation, is it possible to do that?
Not the way to go. You are likely to pay full list price if you turn up to go. That is the standard practice, most ferries are not full so they do not need to sell last minute bookings cheap.
Taking it that you are on here as you are taking car and van the best bet is Dover - Dunkerque probably , on Norfolk Line you can travel out and back for around
Had a quick look at www.ferrycheap.com (or .co.uk cant remember)Excellent price for car against brittany direct but didn't get round to trying price for outfit.