Oh wow, so many replies!!! I didn't get any email notifications - thank you SOO much everyone!!
It seems fairly obvious that we should just get a newborn one!! That makes life a lot easier (and cheaper!!).
@teaandbiscuitsforme Thanks for such a comprehensive answer, so helpful! My DH and I are both over 180cm tall, so I suspect we'll have a tall kid! Very good point about needing to accommodate that possbility. I don't have a Facebook account anymore, but that group does sound good... may make a short term account just for baby crap!
@ShipShop I had no idea you could attach with just a seatbelt rather than an isofix! That's great to know, means we have a lot more options (and saves more money!)
@RippleEffects Ideally not, as it's a filthy stinking diesel and I feel so guilty taking it into the city, but I think it's less of an issue if we get a newborn car seat, as everyone suggests we do (correctly!)
@Elektro The Pioneer is one of the prams we're considering! It will fit in the rear footwell of the tiny car if we take the wheels off, but not in the boot. I dare say it would be easier getting it in the rear anyway, as I wouldn't have to lift it over the boot lip.
@NapQueen I totally didn't think of that - that the car seat actually comes out and you get the kid in and out of it indoors. I'm going to be soooo bad at this parenting malarkey lol. Thanks for the Armodillo recommendation - I'll check it out!
@Spam88 You're totally right, definitely more sensible to wait and see what we need in a year's time. We may actually cut down our cross-country travels - at the minute I'm assuming we'll keep the frequency relatively similar but maybe we'll just get everyone to come to us instead 
@Semilunar22 I didn't think about head position at all - that's a really good point! I've just looked up the igemm carrier and it looks great, thanks for the recommendation!!
OK folks, thanks again - decision made!! Luckily both cars have isofix points so it should be simple to just get the one carrier. Now back to the internet to look at newborn carriers, though I am taken by the Joie already!!
I am so grateful for your wisdom - hope you all have a lovely day, despite the snow!