Net Debt is the sum of all short term debt, and notes payables, Long Term debt and preferred equity minus the total cash and equivalents and short term investments. We calculate this Trailing Twelve Month (TTM) basis, i.e. based on the most recent period (e.g. interims, if available, otherwise the last reported annual).
If we take the example of Berendsen, as of January 2013, the Net Debt is calculated using the latest interims. The NetDebt is therefore £507.1m based on i) Total Interim Debt of £584.3m and ii) Cash and Short Term Investments of £77.2m.