The problem is beyond the waste and the first run as if I disconect the run of pipe from the waste it is full of water so the issue lies between the first 90 degree bend and the drop to the soil pipe.
Thanks
New shower tray installed raised to give reasonable fall. Wast runs to a normal trap from there to a 90 degree right hand then to a 90 degree down through floor to soil pipe. Water starts to run away fine then stops and doesn't move for ten fifteen mins. Piping is the same as before just new...