11. Goodwill and other intangible assets, net

Purchased goodwill
Computer
software
$m
US
$m
UK
$m
Total
goodwill(1)
$m
Cost:
At 29 January 2005 6.2 674.1 228.4 902.5
Additions 9.9
Translation differences (0.4) (14.4) (14.4)
At 28 January 2006 15.7 674.1 214.0 888.1
Additions 7.5
Disposals (0.2)
Transfers 0.6
Translation differences 0.9 24.2 24.2
At 3 February 2007 24.5 674.1 238.2 912.3
Additions 11.3
Transfers (2.6)
Translation differences
At 2 February 2008 33.2 674.1 238.2 912.3
Amortization:
At 29 January 2005 4.0 266.1 86.5 352.6
Charged in period 2.2
Translation differences (0.4) (5.4) (5.4)
At 28 January 2006 5.8 266.1 81.1 347.2
Charged in period 2.4
Disposals (0.2)
Transfers 0.2
Translation differences 0.6 9.1 9.1
At 3 February 2007 8.8 266.1 90.2 356.3
Charged in period 4.7
Transfers (2.2)
Translation differences (0.1)
At 2 February 2008 11.2 266.1 90.2 356.3
Net book value:
At 2 February 2008 22.0 408.0 148.0 556.0
At 3 February 2007 15.7 408.0 148.0 556.0
At 28 January 2006 9.9 408.0 132.9 540.9

(1) Goodwill of $520.2 million net of accumulated amortization arose on acquisitions prior to 1991.

The reporting units for the purpose of goodwill impairment testing are the US and UK operating segments. In 2007/08, 2006/07 and 2005/06, the Group performed the required impairment tests of goodwill and determined that there was no impairment.

Computer software is stated at cost less accumulated amortization. Amortization charges are recorded in selling, general and administrative expenses in the income statement. The weighted average amortization period for additions in the period is 4.1 years.

Management estimates that the annual amortization charge in respect of computer software held at 2 February 2008 for each of the next five fiscal years will be as follows:

$m
2008/09 6.5
2009/10 6.1
2010/11 4.3
2011/12 2.9
2012/13 1.0

Estimated amortization expense can be affected by various factors including future acquisitions and disposals.