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Based on historic default rates, the Company believes that no impairment allowance is necessary in respect of trade receivables not past due. 

## (e) Liquidity risk 

## Exposure to liquidity risk 

The following are the contractual maturities of financial liabilities, including estimated interest payments and excluding the impact of netting agreements: 

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* accrued post-employment benefit liability of $3.1 million, accrued interest of $7.1 million on unsecured senior notes and accrued interest of $0.1 million on derivatives are reported in trade and other payables in 2011 (2010: $2.9 million, $8.1 million and $1.9 million, respectively) 

The following tables indicate the periods in which the cash flows associated with derivatives that are cash flow hedges are expected to impact the income statement: 

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