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Employee benefits - Weighted average duration of benefit obligations (Details)
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
U.K.    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 13 years 15 years
France    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 13 years 13 years
Germany    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 10 years 11 years
Other    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 9 years 10 years