You had overboost with a spark issue? That is highly unlikely mate, the things you should be looking for as the wastegate is controlled by the air produced by the turbo are, blockages, kinks or splits in the complete wastegate pipework between the turbo and the wategate solenoid, you can test for wastegate failure by plugging the outlet nipple directly to your wastegate actuator nipple and going for a run the turbo should only run wastegate spring pressure in this setup if it runs higher than this to overboost your actuator is goosed, or you have boost creep. Another thing that would cause overboost is if the restrictor pill in the short piece of pipe between the 't' piece and the turbo is the wrong size (too small) or incorrectly situated ie on the other side of the 't' piece.
If the wastegate solenoid was to be stuck open this might also cause the car to overboost, it might be worth giving it a clean anyway.
Let us know how you get on
Cheers Iain