As stated, fit a new plug (MK would be good).

Plug is most likely due to a loose screw terminal or
partly broken wire causing the wire to overheat. Plug
pins are large CSA to minimise heating, so oxidation
slowly occurs over time creating the "fish smell".

Socket is least likely, but similarly possible due to a
loose screw terminal, plaster in terminals or a screw
biting insulation rather than conductor & working loose.
A socket can suffer plug contact oxidation or simply not
make good contact with a plug. So if the problem does
re-occur it may be worth replacing the socket also.
--
JSB.