Initial google searching proved inconclusive - some forums stated that the wheels would fit direct without modification, other sites claimed modification would be required.
Further research showed that the Hub of the e70 X5 has a centre bore of 74.1mm whilst the centre bore of the e53 hub is 72.6mm - thus the wheels from the e70 are 1.5mm too big in the hub bore. There is only a small difference and it would appear some people have fitted e70 wheels directly onto the e53 without issue though even a small amount of play could lead to wheel wobble. As such to 'fill the gap' Hub Centric rings are needed.
I sourced my Hub Centric Rings from EBAY; from thenutsandboltshop - would appear from the packaging to be 'Rochford Tyre and Auto Centre' - the set of 4 Hub Centric Rings was £7.95 with free P&P and they arrived next day - so brilliant service and very cheap too. From their ebay listings would seem to have suitable rings for all BMW models.
All in all very happy have changed the wheels, am really pleased with the new (though of course would still love 21s!!!!). See pics below for before and after.
Before
After
looks good and some great info there for others aswell 