You have a simple mis-match between a field name in the source and the field name in the DATABASE field (i.e. Format([Total to be paid],'£#,##0') AS `Total claim` should be Format([Total claim],'£#,##0') AS `Total claim` or as Format([Total claim],'£#,##0') AS `Total to be paid`).

Your field coding could also be simplified somewhat:
{DATABASE \d "{FILENAME \p}/../list.xls" \s " SELECT Format([Period], 'MMM-YYYY') AS `Period`, [Type], Format([claimed],'£#,##0') AS Claimed, Format([Paid],'£#,##0') AS Paid, Format([fine 50%],'£#,##0') AS `Fine 50%`, Format([Penalties 10%],'£#,##0') AS `Penalties 10%`, Format([Fixed fine],'£#,##0') AS `Fixed fine`, Format([Penalties pid],'£#,##0') AS `Penalties pid`, Format([Total claim],'£#,##0') AS `Total claim`, Format([Interests],' DD-MMM-YY ') AS Interests FROM [Invoices$] WHERE [name] = '{MERGEFIELD name}' ORDER BY [Period]" \l "23" \b "1087" \h}

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]