and in my case, the diode is to stop the siren circuit completing via the ground jump back of the relay when the door opens, and the starter killer wire is not grounded.
ordered a few 1a diodes [guy selling the signal diodes listed a lot of 50 for a cpl quid also.]
max current flow should only be 500mA -> 300 the siren, and up to 200 for a second relay for later additions so 1a should suffice.
ALSO: found the spec/datasheet for the scr and it worked. until i broke the tiny legs off the thing when removing tempory test circuit to solder into properly [just my luck - lol] - will teach me to be either more careful, or solder wires to the legs to test with next time :P lol