Vans generally ride the road much harder than cars. Things are usually a lot worse when the van is empty, as there isn't a lot of weight to prevent the suspension bouncing up and down. I'd recommend to downsizing to 15-inch wheels. The issue you have is caused by a combination of the larger wheels and the uprated suspension, to take the maximum 850kg payload. Generally, the Berlingo will carry 650-750kg, with the heavy duty model (your one) using tougher springs to cope with the extra weight. Obviously, you benefit from the increased carrying capacity but the downside appears to be a rough ride when the van is empty.