I have an en suite bathroom with a shower sink and toilet that all appear to be connected. Every so often I get a blockage, but this clears itself usually before returning some months later. The symptoms are: When I drain either the shower or sink or flush the toilet, I get bubbles in the other two units. The toilet fills up more than usual on flushing before draining to normal level. The shower fails to drain at all, but does so on using a plunger, clearing quickly. If I fill the sink, it may fail to empty, but does so immediately on using the plunger in the shower. Same with shower/toilet. I have been trying to fill all three and empty simultaneously. If I do this, I get some upward leakage from the small (9" approx) manhole outside, with some paper visible but no smell. Sometimes the water in the toilet bowl discolours after not using for some time, starting at the back. At no time is there any sewage smell from the sink or shower. What is most likely to be the problem, and how do I approach this? Reading various forums has suggested to me that this may be a venting issue, but if it is, I've no idea how to tackle it.