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Share-based payments, Awards granted under the share option plan assumptions (Details) - GBP (£)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00 £ 64.00 £ 144.14
Fair value (in pounds per share) £ 33,400 £ 11,950 £ 0
Share option plan [Member] | Range one [Member]      
Option pricing model share options granted [Abstract]      
Exercise price (in pounds per share) £ 43.37    
Fair value (in pounds per share) £ 91,994    
Share option plan [Member] | Range two [Member]      
Option pricing model share options granted [Abstract]      
Exercise price (in pounds per share) £ 120.87    
Fair value (in pounds per share) £ 3,309    
Share option plan [Member] | Range three [Member]      
Option pricing model share options granted [Abstract]      
Exercise price (in pounds per share) £ 150.00    
Fair value (in pounds per share) £ 11,481    
Share option plan [Member] | Range four [Member]      
Option pricing model share options granted [Abstract]      
Exercise price (in pounds per share) £ 64.00    
Fair value (in pounds per share) £ 803,488    
Share option plan, Nov-20 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00    
Exercise price (in pounds per share) 64.00    
Hurdle rate (in pounds per share) £ 0    
Expected volatility 87.00%    
Expected life (years) 3 years    
Risk free rate (0.01%)    
Fair value (in pounds per share) £ 35.00    
Share option plan, Oct-20 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00    
Exercise price (in pounds per share) 64.00    
Hurdle rate (in pounds per share) £ 0    
Expected volatility 87.00%    
Expected life (years) 3 years    
Risk free rate (0.07%)    
Fair value (in pounds per share) £ 35.16    
Share option plan, Jun-20 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00    
Exercise price (in pounds per share) 64.00    
Hurdle rate (in pounds per share) £ 0    
Expected volatility 85.00%    
Expected life (years) 3 years    
Share option plan, Jun-20 [Member] | Bottom of Range [Member]      
Option pricing model share options granted [Abstract]      
Risk free rate 0.02%    
Fair value (in pounds per share) £ 34.30    
Share option plan, Jun-20 [Member] | Top of Range [Member]      
Option pricing model share options granted [Abstract]      
Risk free rate (0.03%)    
Fair value (in pounds per share) £ 34.32    
Share option plan, Apr-20 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00    
Exercise price (in pounds per share) 64.00    
Hurdle rate (in pounds per share) £ 0    
Expected volatility 79.00%    
Expected life (years) 3 years    
Risk free rate 0.03%    
Fair value (in pounds per share) £ 32.394    
Share option plan, May-19 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00    
Exercise price (in pounds per share) 150.00    
Hurdle rate (in pounds per share) £ 0    
Expected volatility 67.00%    
Fair value (in pounds per share) £ 11.95    
Share option plan, May-19 [Member] | Bottom of Range [Member]      
Option pricing model share options granted [Abstract]      
Expected life (years) 1 year 10 months 24 days    
Risk free rate 0.69%    
Share option plan, May-19 [Member] | Top of Range [Member]      
Option pricing model share options granted [Abstract]      
Expected life (years) 3 years    
Risk free rate 0.71%    
Share option plan, Apr-17 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 150.00    
Exercise price (in pounds per share) 150.00    
Hurdle rate (in pounds per share) £ 0    
Expected volatility 65.00%    
Expected life (years) 5 years    
Risk free rate 0.42%    
Fair value (in pounds per share) £ 80.63    
Share option plan, 2016 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 140.00    
Hurdle rate (in pounds per share) £ 0    
Expected volatility 60.00%    
Expected life (years) 5 years    
Share option plan, 2016 [Member] | Bottom of Range [Member]      
Option pricing model share options granted [Abstract]      
Exercise price (in pounds per share) £ 43.37    
Risk free rate 0.62%    
Fair value (in pounds per share) £ 77.16    
Share option plan, 2016 [Member] | Top of Range [Member]      
Option pricing model share options granted [Abstract]      
Exercise price (in pounds per share) £ 150.00    
Risk free rate 1.41%    
Fair value (in pounds per share) £ 107.94    
Growth share plan [Member] | Range one [Member]      
Option pricing model share options granted [Abstract]      
Hurdle rate (in pounds per share) £ 170.00    
Fair value (in pounds per share) £ 43,631    
Growth share plan [Member] | Range two [Member]      
Option pricing model share options granted [Abstract]      
Hurdle rate (in pounds per share) £ 64.00    
Fair value (in pounds per share) £ 19,260    
Growth share plan [Member] | Range three [Member]      
Option pricing model share options granted [Abstract]      
Hurdle rate (in pounds per share) £ 0    
Fair value (in pounds per share) £ 0    
Growth share plan [Member] | Range four [Member]      
Option pricing model share options granted [Abstract]      
Hurdle rate (in pounds per share) £ 0    
Fair value (in pounds per share) £ 0    
Growth Share Plan, Apr-20 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00    
Exercise price (in pounds per share) £ 0    
Expected volatility 91.00%    
Expected life (years) 1 year    
Risk free rate 0.03%    
Growth Share Plan, Apr-20 [Member] | Bottom of Range [Member]      
Option pricing model share options granted [Abstract]      
Hurdle rate (in pounds per share) £ 64.00    
Fair value (in pounds per share) £ 2.12    
Growth Share Plan, Apr-20 [Member] | Top of Range [Member]      
Option pricing model share options granted [Abstract]      
Hurdle rate (in pounds per share) £ 170.00    
Fair value (in pounds per share) £ 7.05    
Growth Share Plan, Jun-20 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 64.00    
Exercise price (in pounds per share) 0    
Hurdle rate (in pounds per share) £ 64.00    
Expected volatility 102.00%    
Expected life (years) 1 year    
Risk free rate (0.02%)    
Fair value (in pounds per share) £ 7.05    
Growth Share Plan, Apr-17 [Member]      
Option pricing model share options granted [Abstract]      
Share price at grant date (in pounds per share) £ 150.00    
Exercise price (in pounds per share) 0    
Hurdle rate (in pounds per share) £ 170.00    
Expected volatility 65.00%    
Expected life (years) 2 years 8 months 12 days    
Risk free rate 0.15%    
Fair value (in pounds per share) £ 58.55