When we purchased the van the sill was broken, rotten and a piece of the sliding door seal had fallen off

Obviously this was allowing water and dirt from the road to get into the sill and the sliding door skin. It needed to be fixed.
To remedy this I purchased a length of channelling as near as I could to the same thickness of the channel on the sill that held the rubber seal in place. I then angle ground the sill flat up to the point where the sill channel had broken, primed it, pop riveted the channel to the sill, then painted it. A secondhand sliding door seal was obtained for £5 and I used just the lower part to replace the entire lower length of the seal. Job done!