Please provide advice on best way to clean a used cot mattress which is in excellent condition, tight fit in cot. Thank you

Red Nose has no specific information about cleaning a used mattress.
A good “dose of sunlight” can be efficient.
Is this a mattress you have used before or is it a second hand one you have bought?
If it’s the latter, do you know where/how it has been stored?

Any previously used mattress should have been well stored, firm and clean. If the mattress is damp, torn or mouldy or there is any doubt about its safety, including fit, it should not be used.
https://rednose.org.au/article/second-hand-mattresses