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 |