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Revenue and segmental reporting, Trade receivables, contract assets and contract liabilities (Details) - GBP (£)
£ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Receivables from contracts with customers [abstract]    
Additions £ 453 £ 897
Trade Receivables [Member]    
Receivables from contracts with customers [abstract]    
Beginning balance 1,186 3,600
Additions 4,023 3,431
Deductions (5,209) (5,845)
Ending balance 0 1,186
Contract Assets [Member]    
Receivables from contracts with customers [abstract]    
Beginning balance 424 0
Additions 1,658 424
Deductions 0 0
Ending balance 2,082 424
Deferred Revenue [Member]    
Receivables from contracts with customers [abstract]    
Beginning balance 76,418 98,232
Additions 0 0
Deductions (24,432) (21,814)
Ending balance 51,986 76,418
Contract Liabilities [Member]    
Receivables from contracts with customers [abstract]    
Beginning balance 76,418 98,232
Additions 0 0
Deductions (24,432) (21,814)
Ending balance £ 51,986 £ 76,418