I used these from Paddocks:
Dieselglow heater plugs with wires.(set of 4)
Dieselglow heater plugs with wires.(set of 4) - SERIES
GAM100
£17.75 ex VAT
Modern type wired in Parallel, so if 1 fails the other 3 still work as against the original type which were wired in series, if 1 failed they all stopped working, like a christmas tree.
Alsolute doddle to do.