Can anyone help me with guidance on when you may use a harness/securing points when using a ladder?

Many thanks.

It is a difficult question to quantify because you need to consider the hierarchy of WAH, reasonably practical, your risk assessment and industry standards. That said, it would also be dependant on the task as LawLee suggests. Bet you are glad you asked!

For short duration work, whatever that is deemed to be, a ladder which is secured at the top and bottom may suffice. However, for a more secure ladder you could use a stretch tie around the ladder and anchorerd with a eye bolt, with operative clipped onto a rung with a Karbiner and harness. For example, someone doing cavity wall work this system would be considered the minimum standard for WAH.