Well in the end, as it was around 2230 by the time we actually got off the ferry, I did what was easiest, being my first time driving alone on the continent, and wanting to get to sleep quickly, I drove off the ferry, and followed the lorries, and turned left before going out of the port. I then got into a bit of a lorry jam, as the park was absolutely jam packed and there were lorries coming out and the one in front of me started backing - oohh - quick into reverse, and hoping the lorry behind me could also reverse, I did a sort of about turn as I had space, and took the right side of the barrier, which took you around to the reception parking..... the sign was bent and all I could see was 3m - and remembering the post earlier, I thought, oh gosh, I will go very slow and keep everything crossed..... managed to get thru without any mishaps, (we are 3.15m) and parked up, popped into the reception desk and checked it would be ok to stop overnight there. Fine they said.
So quickly went to bed. It was a bit noisy, but to be honest, it didn't bother us, we got off to sleep quickly and next morning, stopped to look in the duty free shop, to see what they had and prices (for our return) and then drove out, and guess what the barrier, going out said 3.3m - and when we actually arrived back there at the end of the trip in daylight, that bent sign does actually say 3.3m - so we had been alright all along.
Actually we found it quite interesting watching the comings and goings of these juggernauts, and the difficulties they got into due to some unthoughtful parking of other lorries.
I would park there again, given the time of night - especially as I had had (what for me was a long drive to Dover from North Devon that day). My brain needed time to get around the drive on the right bit.... which was OK next morning.
Hope that helps, we will be there again Sept 14th for the 1700 crossing.