Custom Format Measure column to add brackets/parentheses to negative numbers

Hi all,

My finance department has asked that the currency columns be format from -£5,000 to (£5,000)

I've tried a few options but I can't get this to work when the column is a measure...

Variance =
FORMAT (
    CALCULATE (
        SUM ( Margin[Cont Margin] )
            - SUM ( Margin[Cont Budget] ),
        FILTER ( ALL ( 'Calendar' ), 'Calendar'[Week] <= MAX ( 'Calendar'[Week] ) )
    ),
    "£#,##0;(£#,##0)"
)

The column is a calculating a cumulative variance over the weeks which needs formatting. The formatting works for any other column except a measure...

Any tips would be great, cheers