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Financial instruments - Additional Information (Detail)
£ in Millions
 6 Months Ended
Jun. 30, 2019
GBP (£)
Disclosure of detailed information about financial instruments [line items]  
Weighted average growth rate 22.60%
Risk adjusted discount rate 1.50%
An increase or decrease in the multiple of one times revenue [member]  
Disclosure of detailed information about financial instruments [line items]  
Increase or decrease in fair value of other investments £ 55.9
Increase decrease on Income statement due to change in fair value of other investment 12.6
Increase decrease on equity due to change in fair value of other investment 43.3
One percentage point increase or decrease [member]  
Disclosure of detailed information about financial instruments [line items]  
Increase in combined liabilities due to earn out and put option arrangement 7.4
Decrease in combined liabilities due to earn out and put option arrangement 12.6
0.5 percentage point increase or decrease [member]  
Disclosure of detailed information about financial instruments [line items]  
Increase in combined liabilities due to earn out and put option arrangement 6.8
Decrease in combined liabilities due to earn out and put option arrangement £ 6.6