TQOTW: RELIEF FOR LOSSES ON LOANS TO TRADERS

My client, an individual, recently made me aware of a loan he had made to a trading company which has since ceased to trade and become insolvent. As a result there is a substantial sum left owing to him that he will never recover. I am aware that capital loss relief may be available in such circumstances and that the relief may be claimed in an earlier tax year. Can you please remind me of the conditions and limits in terms of how far the relief may be carried-back?