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Revenue and segmental reporting, Deferred revenue (Details) - GBP (£)
£ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue and segmental reporting [Abstract]      
Revenue recognised related to performance obligations £ 0 £ 0 £ 0
Deferred revenue [Abstract]      
Current deferred revenue (Note 23) 27,118 28,457  
Non-current deferred revenue (Note 19) 24,868 47,961  
Total deferred revenue 51,986 76,418  
Deferred revenue held, included in current deferred revenue £ 7,361 £ 3,132