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Employee benefits - Weighted average duration of benefit obligations (Details)
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
U.K.    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 12 years 13 years
France    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 16 years 17 years
GERMANY    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 9 years 10 years
Other    
Disclosure of defined benefit plans [line items]    
Weighted average duration of defined benefit obligation 9 years 9 years