Bulk of Deposit not Refunded

Moving into a property rented out on behalf of the owner by Pompa, Bedfordview; not a single item on my pre-checklist was ever repaired over a period of about three years. During that time I added to the value of the property by upgrading / installing certain items. Upon ending the lease, Pompa refused to refund the bulk of my deposit citing the very items I listed on the signed preoccupation checklist; with minor exceptions. When conversing with Andrew about this he conceded that it was very difficult to get a deposit back from that particular landlord. To add insult to injury Pompa have not arrived for a mediation with the Rental Tribunal on this matter. While this has severely impacted my financial circumstances; I am concerned for other tenants who sign a lease with Pompa on behalf of the same landlord without knowing that their deposits have the potential to be unfairly held back.