Private Eye of October 14 (p. 29) sets it out very well, after the Treasury published the details of the deal:
The idea is that in return for 20-25% of undeclared British accounts that can be identified, the Swiss will keep their secrecy.
This doesn’t apply to “assets of associations of persons, asset structures, trusts or foundations, if it is not possible to ascertain the beneficial ownership of such assets”. As Private Eye puts it, “this is precisely what the Swiss specialise in”.
Also, there’s an 18-month window before this comes in – May 2013, so even the dim accountholder who used their own name has plenty of time to move the money. The Swiss do have to specify the number of accounts shifted and the top ten tax haven destinations – but not how much money has been moved, or by whom!
Also, the UK has agreed not to make public the data collected… so we won’t know, and it is FOI-proof!