Thanks for the comments guys.
The professional advice, as expected from a bit of diagram reading, was that the flange is indeed sealed by a rubber O ring and a tight little begga it has made.
Using stilsons I was only able to partly rotate the backing flange but never got close to moving it off the steel gaskets.
I ended up grinding one side of the bolt head and managed to reinsert the bolt to get it all fitted up. I figured the interference fit of these bolts, the remaining 3/4 of the bolt head supported by 3 other bolts wasn't going to see an issues down the track. Would have preferred to have partly pulled the backing flange but it wasn't budging. So I ended with this view:
Had to refill the diff and a few other bits and piece and ended the night thus:
Which translates to very happy.
Mind you I've yet to tighten the output shafts up again - DoH
But only hand tight hard given the cone nuts in use I was told. That I can do and not flock it up!!
Cheers