I've recently bought the latest Traveller portacot. The mattress seems very firm, am I able to purchase an additional mattress to soften it?

This is something we strongly advise against. Our traveller mattress is tested and conforms to the latest standards against SIDS and SUDI. If you were to add a second mattress, this would potentially make the traveller unsafe, as the second mattress would not fit as securely to the frame of the traveller and could potentially be a hazard. Please see here (NZ) for additional info on keeping your baby safe during sleep.

http://www.kidshealth.org.nz/keeping-your-baby-safe-during-sleep