Called into a local office block last week to sort out a toilet cistern not filling, they also mentioned that it ponged in the toilet - and it did.
Ground floor toilet, cistern boxed in with those large panels. Removed panel, repair was easy, ball valve full of scale wasn't letting much water in.
Cause of the smell was immediately apparent. Soil pipe had a upright section some 4' high, which had a push fit air admittance valve fitted -
Easy enough to fix, but I wonder what caused it to be blown out so violently? Any ideas?