Thanks for your replies and help John, much appreciated
So today I went under again to have a closer look. Seems like the original plastic clips are fitted to the underside by a sort of serrated pin onto which the clips are pushed, a bit like a rawl plug. However, the pins on mine are gone, corroded away so nothing for the clips to push on to.
My solution, drill a hole in the clip to allow a self tapper to go straight through, drill the underside, then screw the clip back on. Did it with the 2 rearmost ones and, touch wood, should last a bit longer
