.......................and the proof was in the pudding.
I finally received the 6.3v 3300uF caps and have just finished removing the old ones and fitting the new ones. No problems and my soldering was far cleaner than normal too so I was certain there would be no issue in that regard.
Turned on the LE Green Xbox and it booted perfectly first time. Left it on for 10 minutes, played a bit of Timesplitters 2 Arcade just as a final test and all is good.
So I can only come to the conclusion that it was those three caps blowing which were the entire cause of the problem.
Worth noting I think.