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Employee benefits - Principal actuarial assumptions regarding defined benefit plans (Details) - year | Sep. 30, 2025 | Sep. 30, 2024
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U.K. |  |
Disclosure of defined benefit plans [line items] |  |
Discount rate | 576.00% | 5.00%
Future salary increases | 31.00% | 0.31%
Future pension increases | 285.00% | 3.01%
Inflation rate | 306.00% | 3.15%
Longevity at age 65 for current members, Males | 21.4 | 22.1
Longevity at age 65 for current members, Females | 23.6 | 23.9
Longevity at age 45 for current members, Males | 22.7 | 23.5
Longevity at age 45 for current members, Females | 25.1 | 25.4
France |  |
Disclosure of defined benefit plans [line items] |  |
Discount rate | 370.00% | 3.33%
Future salary increases | 275.00% | 4.10%
Future pension increases | 0.00% | 0.00%
Inflation rate | 200.00% | 2.00%
GERMANY |  |
Disclosure of defined benefit plans [line items] |  |
Discount rate | 370.00% | 3.33%
Future salary increases | 250.00% | 2.50%
Future pension increases | 210.00% | 2.10%
Inflation rate | 200.00% | 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 | 24.0
Longevity at age 45 for current members, Females | 27.0 | 27.0
Other |  |
Disclosure of defined benefit plans [line items] |  |
Discount rate | 509.00% | 5.06%
Future salary increases | 230.00% | 2.74%
Future pension increases | 29.00% | 0.31%
Inflation rate | 349.00% | 3.44%

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- Definition The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [domain]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2025-01-01 -Paragraph 144 -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2025-en-r&anchor=para_144&doctype=Standard -URIDate 2025-03-27 + Details
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- Definition The expected rate of inflation that is used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [domain]; Defined benefit obligation, at present value; Actuarial assumptions [domain]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2025-01-01 -Paragraph 144 -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2025-en-r&anchor=para_144&doctype=Standard -URIDate 2025-03-27 + Details
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- Definition The expected rate of increases in pension that is used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [domain]; Defined benefit obligation, at present value; Actuarial assumptions [domain]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2025-01-01 -Paragraph 144 -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2025-en-r&anchor=para_144&doctype=Standard -URIDate 2025-03-27 + Details
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- Definition The expected rate of salary increases used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [domain]; Defined benefit obligation, at present value; Actuarial assumptions [domain]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2025-01-01 -Paragraph 144 -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2025-en-r&anchor=para_144&doctype=Standard -URIDate 2025-03-27 + Details
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