Trouble with main dealers are they employ 17yr old apprentices and one master tech who doesn't get involved with older cars , least thats the case round here anyway .
I run one of these 5.0 litre XFR beasts , love it , but my son works on it so its not been near a main dealer , i will ask him when he gets back if he knows anything about your problem , seems coincidental that's its happened after the injector work , He's had mine in a million bits changing chains and tensioners and guides , new pulley on super charger , VVT parts replaced before the re map , never had water problems though .
How many miles has it done ? cos some things changed in the engine on various years .