The law in Scotland, and I believe also in England & Wales allows for interest to be added to unpaid debts or damages at the statutory rate if 8% simple interest per annum.
You should write as early as possible in the process to inform the transgressor that you intend to add interest.
Worth noting that simple interest is calculated differently to the much more commonly used compound interest, and generally results in a lower number.
One call also add reasonable costs in obtaining the money, for example if court has to be paid for, if bailiffs have to be paid for etc. Worth noting that if the "simple court procedure" is used (Formerly known as "Small claims court") then the expenses that can be claimed for are more limited and closely detailed. Legal advice and representation is specifically excluded. Court fees and Bailiff fees are allowed up to the procedure limit - which varies around the UK.
I hope this helps, and I hope you obtain redress.