My grandson is 14 months old and through the day is sleeping in a low toddler bed for his day naps. The cot mattress is very firm so I have placed a mini jumbuck cot woollen underlay on it. The underlay is breathable and fits very tightly. And with no give in it. The mattress is still firm. What is your view about the safety of this? Many thanks.

Red Nose has the following information about the use of mattress protectors & woolen underlays.
“A soft mattress or sleeping surface can increase the risk of sudden unexpected infant death if baby rolls over onto the tummy. Remove plastic packaging from the mattress and always make sure that the waterproof mattress protector is strong and a tight fit. Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft”
https://rednose.org.au/article/what-is-a-safe-mattress
While your grandson is beyond the risk of Sudden Unexpected Death in Infancy (including SIDS) by definition, there can still be an ongoing risk of fatal sleeping accidents. While this risk is very low, then general recommendation is still to avoid soft mattresses & any loose or padded bedding that might cover head & face.