Newborn babies often don't fit well in any car seat - the 0-4 year ones in general tend to be harder to get a good fit than infant carriers, but even infant carriers are variable in fit. Car seat manufacturers work with a "newborn" dummy which is based on a 6 week old, but 6 week old babies can be a good 1-2kg heavier than they are at birth and about 5-6cm longer. That's a huge difference when they are so tiny - it's a whole clothing size.
You could phone up John Lewis customer service and see what they say, but I think it is possible that they would be reluctant to refund as they won't be able to resell the seat since you have used it. They may offer fit advice if you bring the seat and baby to the store. Perhaps one parent could drive and the other take the baby on the bus? As others have said, you will be able to use this seat when LO has outgrown an infant carrier in any case, or possibly sooner (the 6-week difference, as said).
If you do buy another seat, the Avionaut seats are easily the best although they aren't cheap (Cosmo is not too bad, and does not need the base to fit).
Britax and Maxi Cosi also tend to get good fit from newborn. But I do think that you will find if you wait a couple of weeks, he will fit better in the Joie Juva seat that you have. It will probably also be easier to put the Juva at more of a recline due to it being belt fitted. I would say that you don't need to buy another seat, though if you want to, the above brands are what I would recommend.
If you want to improve support in the Juva seat, you can add some rolled up muslin cloths alongside him - this is a really detailed article which explains how to do that safely. Although it's American, the only part which is different to UK seats is the harness - they have a 5 point harness plus the chest clip whereas UK seats often have a Y-shaped harness for infant carriers. Everything else is fine and applies the same. You don't see this advice a lot for UK seats because most car seats have more structured inserts these days than the Joie Juva. The Juva is a more old fashioned type design.
thecarseatlady.com/newbornheadposition/