data not pulling throught due to relationships

If I was you, I would create a summary table in Excel similar to the table you have in powerbi.  Also, order the table (above) in Excel by date 'signed in programme' so that you can cross reference with the powerbi table.

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In Powerbi, create another table with the actual participant reference (not a count) and 'signed in programme' field.  Work through it by Month comparing it to the excel table.  You might also want to use Data View in Powerbi to filter from the column header for each month.  The No of Records will show at the bottom of the screen.

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Start with the first month. If there are x records in Excel but more than x records in powerbi, narrow the investigation by looking at individual records.

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I notice that there are 3 records in powerbi shown in the table which don't have a 'signed in programme' value.  Are they included in the Excel total?

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This shouldn't take too long.  You only need to find 1 or 2 records which are shown in powerbi but not excel and you investigate those to see where the problem lies.

Once you find the 'problem' , invsetigate if all the 'extra' records are caused by the same issue.