No, honestly, I'm not making this up...

My girlfriend lives in a townhouse, and the mid-level toilet (WC and hand
basin) has started to smell. It's difficult to locate, but it smells
slightly fishy - not the sort of smell I would expect from a leaking pipe,
etc. I've tried blocking off the extractor, plugged the sink and a plumber
has removed the toilet and capped the waste pipe - but the smell remains,
and we can't seem to get rid of it.

Any ideas on what might be causing this, please.......?

Thanks

Martin