Wow...many thanks guys...great responses - Malc, Mike, Steve and Andy.
This afternoon both Lexus Manchester and Lexus Stockport confirmed that this "063" is the correct one for my car using the reg and chassis number.
I actually bought a used battery 6 months ago, that was an "068" and came from a Lexus breakers, and the gentleman told me it was from a MkIV LS400, and was thus a bit smaller. Even this "068" was too small for the battery tray, and a result the battery clamp wouldn't fit so has had to be cable-tied down.
I just want the right one really, one that the battery clamp fits on, and that should be the correct one! Hmm I shall do some thinking.
I think I will go to Lexus Stockport, buy the one they say, then get them to put it on, and if the battery clamp won't fit, we will know. I must admit I am swaying towards an 075 as my mechanic did show me how the 068 that i bought was too small for the tray, and it also sounds like an 075 would be more suited to a 4.0 V8. But then again, the dealer should know for sure...and also Andy confirmed what the dealer said. Ohhh I don't know...