reeimbursment for a broken laptop unit

a laptop unit with 8.1 software no longer operated properly after one week of use .<br> took it back after 1 month and asked to be reimbursed .<br> they said that would not be a problem as I had 6 months time to do that .<br> I supp**** them with the original purchase invoice and they took the unit back .<br> They promised that I would be sorted out after 2 weeks .<br> After 2.5 weeks I went back to find out what had happened since .<br> They said nothing had happened as they lost my copy of the original purchase slip .<br> They said I now need to supply them with another copy of that slip .<br>