4. Investments

Shares
£m
Cost
At 1 January 2017397.5
At 31 December 2017397.5
Provision for impairment
At 1 January 20176.6
At 31 December 20176.6
Net book value
At 31 December 2017390.9
At 31 December 2016390.9

The following subsidiaries have taken advantage of an exemption from audit under section 479A of the Companies Act 2006. As the ultimate parent, Bodycote plc has provided a statutory guarantee for any outstanding liabilities of this business. All subsidiary undertakings have been included in the consolidation.

Bodycote Heat Treatments Limited

Bodycote Surface Technology Limited

Bodycote HIP Limited

Bodycote America Finance Limited

Bodycote America Treasury Limited

Bodycote Finance Limited

Bodycote Finance UK Limited

Bodycote International Limited

Bodycote Investments Limited

Bodycote Nominees No. 1 Limited

Bodycote Pension Trustees Limited

Bodycote HIP Germany Limited

Bodycote Treasury Services Limited

Bodycote Thermal Processing Mexico Limited