with the SIDS reccomendations to only ever sleep baby on their back, it is more common for the 'flat head' syndrome.
Babys heads are capable of mouding for a few months, so it can go back into shape naturally.
like Tinks said, if you notice bub always has their head turned a certain way in the cot (common if you are in the same room) sleep them down the other end (turned around) so they will then turn and look the other way, and give that part of their head a rest.
Plus lots of tummy time when they are awake. not only good for upper body strength and control, but also for keeping them off the back of their head.

At times those helmets are ness. they are avaliable through chiropractors, and sometimes some babies do need them. MACH can connect you to some providors and specilists who you can talk to about them. sorry i dont actually know much more specifically about them.

good luck with it.