Like a previous reviewer said, the good reviews are due to the gift vouchers they offer people who have just moved in to the property. These people have clearly had no experience with the agency and have certainly not moved out yet.
The employees of D&H are unprofessional and in many cases uneducated. Dont expect to get through to them easily on the phone. Whoever you do get through to will tell you someone else deals with your issue and they will no doubt be conveniently "in a meeting".
Big warning on your deposit - dont expect to see it all again. They charge a huge cleaning fee at the end regardless of how clean you leave the flat and a check out fee (they do nothing for this) which you will not be told about before you sign the contact. I would not normally go to the effort to write a review but feel strongly enough to warn people about this agency.