When their system vat file starts getting close to the 2Gb limit.
Take a backup. Could take a while to process so get them to do it at a weekend.
Go to Reporting, EC VAT, Reports, ESL Report - enter a date to clear up to, then click in Commit Data to ESL.
you must also enter a branch ID and Contact name - make this up if you don't know. Click OK. Choose Publish to screen. You might have to ESC a few times to get through all the prints. At the very end it will tell you that there are validation errors, publish them, press ESC and then when asked "do you want to commit" say yes.
Then go into Administration, Utilities, Reorganise System Files, "Clear VAT file" - delete VAT up to? - remember VAT rules.