well i'm still fiddle-diddling as we speak but i think i've hit the jackpot as to what's going on now.
i wish i could say the mix post-tinkering is as interesting as it is straight off the disc but it isn't, its still nice though and sounds much better balanced with the overloud FL, RR & LFE all dialled back and the other channels level adjusted.
tbh i smelt a rat (or rather spotted one!) as soon as i saw a rear channel with so little going on in it! the penny dropped, there's no lead vocal reberb in there (ch.4 is all low bass content, that's definitely the LFE present and correct) so Channel 5 must be the Centre!!
then the channels
with lead vocal reverb in them (2 & 3) revealed themselves as Rears (i'm going on Ch.2 as Left Rear and Ch.3 Right Rear)...
..and the originally overloud Rear Right (creating a weird kind of diagonal Centre in the middle of the room) not only looks like a Front/Main channel, it is!
so to me (as was suggested on here years ago) it was indeed a bit of a screw up.. not the mix itself but of channel assignment on the disc and then i'm guessing somebody (mastering engineer or authoring house maybe?) thought "gosh this doesn't sound right! lets boost the Fronts and LFE and no-one will notice!"
there is an upside to all this, in that i'm really listening intently to this for possibly the first time ever - and greatly enjoying it, its a lovely album i never appreciated before and dismissed with a QQ "7" because the mix didn't grab me.. now i know why!