04 plate came out in march 04, then 54 came out in september 04, then 05 comes out in march 05 and runs til september, which (you guessed it) gets replaced by 55 plates

that cant be right if he says it's an 04 registered in 05 You could have a 54 registered in 05 though, if it was registered between 1st Jan and 1st March

If it was a private plate i'm sure you can't use a prefix newer than the actual year of the car so it cant be a cherished number either. Not entirely sure on this rule, i knew it used to apply with the old style reg's but not sure about the new xx04 xxx ones