Bloody hell Rich, I get confused by your directions and I go to Weymouth alot....
My advice is down the hill passing the elbow bend, next roundabout turn right, next roundabout bear left, now you should see the river on your left, next roundabout turn left over the river, with Macdonald and KFC on your left, take right at this roundabout, over some tram/rail lines on the road, the tram line will take you all the way to the slipway which is on your right. If if helps, an Iceland van always park in the car park on a Sunday morning.