Retirement benefits (Tables)

12 Months Ended

Dec. 31, 2023

Disclosure of defined benefit plans [abstract]

Defined Benefit Plan

	Net defined benefit obligation
	2023 $m			2022 $m			2021 $m
At 1 January		66			92			103
Recognised in profit or loss
Interest expense		3			2			2
		3			2			2
Recognised in other comprehensive income
Actuarial loss/(gain) arising from changes in:
Demographic assumptions		(1	)		(1	)		(3	)
Financial assumptions		2			(22	)		(3	)
Experience adjustments		1			2			(1	)
Re-measurement loss/(gain)		2			(21	)		(7	)
Exchange adjustments		–			(2	)		(1	)
		2			(23	)		(8	)
Other
Group contributions		(5	)		(5	)		(5	)
		(5	)		(5	)		(5	)
At 31 December		66			66			92
Comprising:
UK plan		19			18			30
US plans		34			35			45
US post-retirement plan		13			13			17
		66			66			92

Summary of Principal Financial Assumptions Used to Determine Benefit Obligations

The principal financial assumptions used by the actuaries to determine the defined benefit obligations are:

	2023 %		2022 %		2021 %
UK plan only:
Pension increases		3.1		3.2		3.4
Inflation rate		3.1		3.2		3.4
Discount rate:
UK plan		4.8		5.0		1.8
US plans		4.7		4.9		2.4
US post-retirement plan		4.7		4.9		2.4
US healthcare cost trend rate assumed for the next year:
Pre-65 (ultimate rate reached in 2034)		7.8		6.9		6.2
Post-65 (ultimate rate reached in 2034)		8.6		7.3		6.5
Ultimate rate that the cost rate trends to		4.5		4.5		4.5

Defined Benefit Plans - Life Expectancy Assumptions at Retirement Age

The assumptions applied to the UK plan and US plans for life expectancy at retirement age are as follows:

		UK						US
		2023 years		2022 years		2021 years		2023 years		2022 years		2021 years
Current pensioners at 65a	– male		23		24		24		22		22		22
	– female		25		26		26		23		23		23
Future pensioners at 65b	– male		23		25		25		23		23		23
	– female		25		27		28		25		25		25

a  Relates to assumptions based on longevity following retirement at the end of the reporting period.

b Relates to assumptions based on longevity relating to an employee retiring in 2043.

Defined Benefit Plans - Sensitivity Analysis of Actuarial Assumptions

The sensitivity analysis below relates to the increase/(decrease) in the benefit obligation and is based on extrapolating reasonable changes in these assumptions, using year-end conditions and assuming no interdependency between the assumptions:

		2023 $m			2022 $m
Discount rate	1% decrease		6			7
	1% increase		(6	)		(5	)
Inflation rate	0.25% decrease		(1	)		(1	)
	0.25% increase		1			1
Mortality rate	One-year increase		3			3
Healthcare costs trend rate	1% decrease		(1	)		(1	)
	1% increase		1			1

Summary of Estimated Future Benefit Payments

Estimated future benefit payments 

	2023 $m		2022 $m
Within one year		5		5
Between one and five years		21		20
More than five years		86		89
		112		114

	2023   years		2022   years
UK plan		13.0		14.0
US plans		7.5		7.6
US post-retirement plan		8.0		8.0