Rorry

I have the same trap as the OP - could someone say what the water level in the odour trap should be? Ours, installed 3yrs ago in a little used ground floor wetroom smelled a couple of times over 3yrs but now I've found that if I fill the odour trap most of the water gets sucked away within a minute leaving just enough to make a seal. Then over a couple of days of using the toilet (gets used quite a lot) enough water goes to break the seal, and the smell is horrendous.