Picking data out of different columns

The purpose of the attached workbook is to calculate the average number of days a customer takes to pay their invoices. There are two tabs - Invoices and Payments. I am trying to calculate the number of days between Invoices "Sales Date" and Payments "Settlement Date".

The Payments tab is raw data exported from Attache financial system as CSV. It is the best I can get out of the system but the columns are all over the place.

I thought maybe from the Invoices tab I could search a row on the Payments tab to match the invoice number, find the cell containing the phrase "Invoice Number" then pick up the date from the following cell which contains the Settlement Date . This would be straight forward if the data from the CSV file consistently landed in the same column in the Payments worksheet, but it doesn't. As you can see, the phrase "Invoice Number" might appear in any one of several different columns.

I would be most grateful if anyone suggest a method for this lookup please.