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Financial instruments - Additional Information (Detail)
£ in Millions
Jun. 30, 2018
GBP (£)
Disclosure of detailed information about financial instruments [line items]  
Weighted average growth rate 23.80%
Risk adjusted discount rate 2.40%
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 £ 8.9
Decrease in combined liabilities due to earn out and put option arrangement 8.7
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 6.6
Decrease in combined liabilities due to earn out and put option arrangement £ 6.9