Had 2 storey extension built over last year involving 2 new bathrooms and thus lots of messing with drains and sewers. For a few months now the downstairs toilet has been showing a worrying tendency for the bowl to occasionally fill quite high with water when flushing. Most of the time its fine though. Then today it very nearly overflowed. Can anyone tell me if this is the fault of the toilet itself or is it likely to be a bigger problem with sewer pipes? Is there something we can do ourselves or should I call a plumber pronto? Thanks.