I have a small plumbing problem in my ensuite. Whenever we flush the toilet, water starts gurgling and comes up through the shower and sink. recently, the shower has started to fill up when its being used, the water doesn't drain away quick enough. when the toilet is flushed, some shower water and some toilet water come up into the shower and then drain away. This morning i had to wash out the shower before i would get in it and when i was doing this the water didn't drain away for ages. when it did drain i got in and had my shower and had no problems, the shower tray didn't fill up with water. About half an hour later, i flushed the toilet and the toilet water came up through the shower and sink again. what could be causing this? I've tried washing soda and caustic soda and they've made no difference.