Sniff closely any electrical fittings - with the passage of time the plastic begins to age - a chemical change - and breaks down - often in old lamp holders - anywhere where there's heat...
From a previous Fishy Smell problem...
"Aliphatic amines have a fishy smell and are liberated when plastic electrical fittings get over-warm and particularly as they age.
Typically, 13-amp plugs are in fact unable properly to handle 13 amp loads (i.e.3Kw) and will always get warm if handling such loads. Sooner or later they will start to smell as will plastic lamp holders fitted with high-wattage bulbs.
Alan Sutton (C.Chem., FRSC)"
http://www.howtomendit.com/answers.php?id=4106
Peccavi
November 2009