I'll stand by the numbers.
If you're interested, here's a link to the Treasury's Red Book with all 104 glorious pages. The Red Book sets out the government's policies, measures and (most importantly here) costings. The costs are now reviewed by the independent Office for Budget Responsibility, so are generally free from political bias or posturing.
cdn.hm-treasury.gov.uk/2011budget_complete.pdf
If you turn to table 2.1 on page 42, you can see that the fifth measure is full reform of the CFC rules, which is what may or may not bring WPP back to the UK. The lost tax is estimated to be:
2011/12 - nil (because the changes won't have been enacted yet)
12/13 - £210m
13/14 - £540m
14/15 - £770m
15/16 - £840m
Of course, that's not just WPP - that's the tax lost from every single company in the UK that would otherwise have paid CFC tax.
The other interesting line for this discussion is the tax lost as a result of cutting the headline rate of corporation tax. Those are also big numbers, and are pretty much being paid for by the bank levy.
FWIW, the Red Book is the first thing I turn to when I analyse the effects of the Budget. Because it tells me at a glance which measures the Treasury think are important (not necessarily the ones that the Chancellor has mentioned in his speech.....).
Enjoy!