I need a mattress only for occasion use.
If you need a cot mattress for just occasional use, perhaps for when the family visit, then you could get away with just a 7cm (3") thick one, like the Airflow. Sometimes when you purchase a cot and get a free mattress the mattress supplied with your cot is a 7cm thick mattress!

I need a cot mattress for a Cot bed
If you are going to use a cot mattress for more than 2 years it is wise to get a more substantial mattress, one that is sprung or with a better quality foam such as CMHR35. As your child gets heavier and more active the cot-bed mattress needs to be able to cope with the extra weight and stress.

I want a cot-mattress that is anti-allergenic
An anti allergenic cover like Safetex will offer maximum protection against dust mites, because it is washable at 60°c. If a coveris not washable at 60°c then it cannot demmed to be anti-allerganic once it has been used.

Should I have a softer cot mattress when my baby is first born.
If you wish to have a softer mattress for your newborn baby look at the reassurance cot mattress which is reversible. It has a soft layer on one side and a firmer foam on the other. You can have the choice of whether to use the breathable moisture resistant Trendtex cover or the soft Hi-Grade PVC cover, a choice of 4 options

I want to avoid the risk of harmful bacteria build up on my cot mattress
In order to keep your cot mattress free of bacteria the cover needs to be moisture resistant and washable, so no body fluids get on to the foam. It is an advantage to have a cover that is able to be wiped clean or preferably machine washable at 60 degrees. PVC will protect your foam mattress from bacteria, however PVC is is not a breathable fabric.and also contains "phalates". there is some debate about the affect of phalates on young babies so we have banned PVC from all our Cot Mattresses