I'm probably at risk of telling you Girls and Boys how to suck eggs? I've got an XKR with the ol "resricted performance" message. After consulting with various honest but non Jag fixing chaps. I was advised that if one coil set goes then replacing all 8 (in my case) was the way forward. Those Bad Boys are coming out around the £80+vat touch, each! I have been told by a fellow called Glen at Jaguar and Aston Martin Specialist Ltd that is not the case and only the faulty one would need replacing. Another little footnote to this is that, the diagnostic machine does not pick up a faulty plug and will show as a faulty coil set. So, applying my sometimes flawed logic, a potential £768.00 plus labour fix could be a simple case of replacing a spark plug!!