Newborn insert for car seat

Hi first time mom who has no idea what she needs to be buying here...

So we bought a second hand travel system with car seat and carry cot as we are on a budget but the car seat hasn’t come with a newborn insert (if that’s even what it’s called?). It’s a Hauck travel system but that’s all I can work out as I don’t think they sell this particular system anymore so not even sure what the car seat is called. I’ve put some pictures below of the car seat to give you an idea but has anyone got any recommendations for what newborn insert I should buy as I obviously need something to cushion/pad out the seat?

Any advice is welcomed and I am in the UK but I’d of thought with the help of Amazon I could get most products

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