Security deposit

We have tried countless times to get our security deposit back from this property agency, but with no luck. When we left the property apartment of Sunnyrock flat Unit 206 in Sunnyside, we were told we will not get our security deposit because we left the bedroom walls dirty, the concrete ceiling had mould and the bathroom basin was cracked. We accept that the walls were dirty and the basin was cracked, but the mould on the ceiling built up naturally, and we had nothing to do with that. We then asked for the breakdown of the costing for the repairs and it was R6500.00. We have been communicating with Dirk de Bruyn who is the building inspector at FPT that we are unhappy with their invoice and that we require a proper breakdown of the costs for painting, and replacement of the basin but he would just send us the same invoice over and over. We asked for an email for his (Dirk) senior in order to escalate this claim. This is the email that he gave us; ********** and there has been no response since. We feel that the costing is unreasonable and we expect some of our security deposit back. We cannot be expected to move on from this without a good and fair resolve.