This excerpt taken from the BP 6-K filed Nov 17, 2005.
Pensions and other postretirement benefits (concluded)
A charge representing the unwinding of the discount on the plan liabilities during the period is included within other finance expense.
A credit representing the expected return on the plan assets during the period is included within other finance expense. This credit is based on the market value of the plan assets, and expected rates of return, at the beginning of the year.
Actuarial gains and losses may result from: differences between the expected return and the actual return on plan assets; differences between the actuarial assumptions underlying the plan liabilities and actual experience during the year; or changes in the actuarial assumptions used in the valuation of the plan liabilities. Actuarial gains and losses, and taxation thereon, are recognized in the statement of recognized income and expenses.
For defined contribution plans, contributions payable for the period are charged to the income statement as an operating expense.