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Share capital and other equity - Reserves (Details) - £ / shares
 12 Months Ended
Mar. 31, 2021
Dec. 31, 2021
Dec. 31, 2020
 [1]
Dec. 31, 2019
Mar. 19, 2021
Ordinary share from discontinued operations basic   £ 0.024 £ 0.010 £ 0.021 [1]  
Ordinary share from discontinued operations dilutive   0.024 £ 0.010 £ 0.021 [1]  
Ordinary shares          
Own shares held         200,000,000
Par value   £ 1      
Delivery of own shares held 200,000,000        
Own shares held | Ordinary shares          
Own shares held   180,937,710      
Delivery of own shares held   19,062,290      
Share awards and options | Own shares held          
Own shares held   15,000,000   16,000,000  
Par value   £ 1      
Delivery of own shares held   1,000,000      
[1] Comparative results have been re-presented from those previously published to reclassify certain items as discontinued operations as described in Note 8 to the consolidated financial statements.