Have the plugs and sockets replaced... very soon... they are not safe...
Heat caused by high current and the passage of time are accelerating the breakdown of the plastic - what you see is a chemical reaction in the plastic - there will be an ammonia / fishy smell.
Wikipedia says of Aliphatic Amine / Triethylamine...
"is liquid at room temperature. It possesses a strong fishy odor reminiscent of ammonia."
An educated contributor here says...
"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)"
Peccavi
November 2009