Level of Water in Both Toilet Pans Dropping

[Karl Berkshire]

2 toilets. 1 upstairs and 1 downstairs. Believe also linked to 2 basins and bath/shower.

The level of the toilets (pans) appears to be decreasing. Not dramatically but noticeably. I have ever checked level of cisterns and appears to be ok.

I think it may be the soil stack that is blocked? Any other ideas as the only way to verify this is to get on the roof as no other access to it. Nothing in attic. Straight pipe.

Toilets flushing ok to pipe.

[lets have a look]

Yes I would suspect a partial blockage,you could try rodding upwards from the manhole or take out a toilet pan and rod that way,if the pipe is plastic then cut it in the loft and rod