2001 XJ 3.2 Knock Sensor!?

[rash_decision]

Hi all! Dragged my Dad's XJ out the garage yesterday for the first time in months! I fitted a battery to it as the old one was dead, and it fired up first time, result! Or so I thought! There are warning lights on the dash, one of which is telling me the car needs an oxygen sensor, which I knew about, but the other light, the red warning light, is lit and the dash is telling me I have restricted power. After plugging the computer into the car it's saying I have a faulty knock sensor! I've never replaced a knock sensor an any car! Is this a Jag common problem, or could it be something silly?? I drove it to the MOT station last night, and if I tried to accelerate it would misfire like nothing on earth! I assume this is down to the knock sensor fault as it drove ok when it just needed an oxygen sensor.