Jonathan,
This code assumes your numbers are in column 3, starting a row 2. It's not as quick as it could possibly be but it works fine.
Regards,
Dan (sat in W1 also)
Sub CorrectFigures()
Dim rngeUsed As Range, rngeCl As Range
Application.ScreenUpdating = False 'Turn off screen updating to speed up macro
'Change this line accordingly. Refers to column 3 and that the figures start
'in row 2 i.e. assumes you have a column header.
Set rngeUsed = Range(Cells(2, 3), Cells(ActiveSheet.UsedRange.Rows.Count, 3))
For Each rngeCl In rngeUsed.Cells
If Right(rngeCl, 1) = "-" Then rngeCl = Val(Left(rngeCl, Len(rngeCl) - 1)) * -1
Next
Application.ScreenUpdating = True
End Sub