Glasgow Canal Bird

I was walking a few days ago on the canal running through Maryhill. At the side of the canal there were reeds and bulrushes. I saw a wren sized bird on one of reeds with a slightly turned up tail and a black bar across the tail (near the end). Before I could take a picture with my mobile it disappeared deep in to the reeds. I'm reasonably knowledgeable on small birds but this threw me. Any suggestions? Thanks,John