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Accounting policies - Additional Information (Detail) - GBP (£)
£ in Millions
 6 Months Ended
Jan. 01, 2018
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Disclosure of initial application of standards or interpretations [line items]        
Work in progress reclassified as trade and other receivables   £ 405.8 £ 401.1 £ 410.6
Retained earnings [member]        
Disclosure of initial application of standards or interpretations [line items]        
Accounting policy change (IFRS 9) £ 407.4 407.4 [1]    
Other reserves [member]        
Disclosure of initial application of standards or interpretations [line items]        
Accounting policy change (IFRS 9) £ (407.4) £ (407.4)    
[1] Retained earnings have been restated for the impact of the adoption of IFRS 15: Revenue from Contracts with Customers, as described in note 2.