I have just tried replacing a LX middle chain ring with an XT on a Race Face crank but the XT is about 1mm different size and wont fit on to the arms. Is that to be expected when the LX fits fine?

It is very possible. Although the BCD will be the same Shimano tend to change the profile of the ring / arm interface.

well I know I had to file some off the inner tabs on a deore ring to make it fit on my XT 760 chainset (and a m8’s), there was a mention the other day that now deore have move to Hollowtech II, the rings fit XT without the filing.
So you experience seems to match this.