Did not change banking details

I asked for my banking details to be changed on the 21 July to be effective for the 1 August debit order. I have confirmation via email that this was done but today I was debited on my old bank account causing me to incur a R100 unpaid debit fee! I will be applying for Home finance shortly and this unpaid debit will cause me to not get the finance. I want the R100 refunded and a letter stating that it was Adt's fault that the unpaid debit order occurred.