This smell plagued one room in my house for 4 years. Floor boards were lifted, walls knocked down, roof examined etc. No professional could tell me the cause. Last week a friend visited and immediately identified the smell as coming from the plastic just above my ceiling light bulb. Every time the light was put on the plastic burnt and the smell of fish pervaded the room.

So, after spending hundreds of pounds attempting to find the source a non professional nose has correctly identified it as coming from a piece of plastic that can be replaced for 50p.

I've now been told by electricians that it is common knowledge in their circles that burnt plastic causes a fishy smell.