Shocking one really, I shut this boiler down a few months back as the flue had come apart due to not being clipped at all, l/lord go original installer back to repair (I'd recommended complete flue replaced as damage to inner flue duct at joints). Got a call again today from tenants as smelling this is what I found.
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POC still escaping, hence the staining, also spilling into the floor void and into the bedroom above, you can see staining above the skirting! L/Lord has seen sense and whole flue is being replaced next week.