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Bank overdrafts, bonds and bank loans - Additional Information (Detail) - GBP (£)
£ in Millions
Dec. 31, 2019
Dec. 31, 2018
Disclosure of bank overdraft bonds and bank loans [line items]    
Carrying amount of bank loans £ 110.4 £ 186.8
Corporate bond [member]    
Disclosure of bank overdraft bonds and bank loans [line items]    
Fair value of corporate bonds £ 4,439.8 £ 5,965.7