relay heater plugs?

i have a relay 1.9d v reg 99/2000.it's getting a bit hard to start in a morning,fine rest of day.i have been told it's probably heater plugs.
also been told there are 2 different types for this model.but after a few searches they all only seem to state one type for this model.i:e same plug from 95 on.how do i find out which plug i need.and is it a fairly straight forward job to do.