There is quite a bit on this if you do a search. I think they have tightened up the rules recently - you used to be able to open the account, transfer £1,000 into it for 3 months and get the miles. Then, a few months later, they would write to you and offer you another 5,000 miles if you made it your main current account.
Last time I looked at this they insisted that you close your existing current account immediately to get the miles. This is a lot more hassle, although of course you could always move back to your current bank in 6 months if you found you really hated NatWest (which is likely).
If you have £100k sitting around, the NatWest International Advantage account is OK, although getting the miles is like pulling teeth. 25k for opening the account and 25k every anniversary. The interest rate is pants, but nothing stops you withdrawing most of the money for 11 months of the year and just topping it up closer to the anniversary.