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Employee benefits - Principal actuarial assumptions regarding defined benefit plans (Details) - year
Sep. 30, 2023
Sep. 30, 2022
U.K.    
Disclosure of defined benefit plans [line items]    
Discount rate 5.60% 4.95%
Future salary increases 0.33% 0.35%
Future pension increases 3.20% 3.30%
Inflation rate 3.39% 3.60%
Longevity at age 65 for current members, Males 22.0 22.0
Longevity at age 65 for current members, Females 23.8 23.8
Longevity at age 45 for current members, Males 23.4 23.3
Longevity at age 45 for current members, Females 25.3 25.3
France    
Disclosure of defined benefit plans [line items]    
Discount rate 4.20% 3.75%
Future salary increases 4.15% 3.77%
Future pension increases 0.00% 0.00%
Inflation rate 2.10% 2.20%
Germany    
Disclosure of defined benefit plans [line items]    
Discount rate 4.06% 4.07%
Future salary increases 2.50% 2.50%
Future pension increases 2.10% 2.10%
Inflation rate 2.00% 2.00%
Longevity at age 65 for current members, Males 21.0 21.0
Longevity at age 65 for current members, Females 24.0 24.0
Longevity at age 45 for current members, Males 24.0 23.0
Longevity at age 45 for current members, Females 26.0 26.0
Other    
Disclosure of defined benefit plans [line items]    
Discount rate 5.62% 5.43%
Future salary increases 2.76% 2.64%
Future pension increases 0.29% 0.42%
Inflation rate 3.46% 3.44%