Mrs Hendo my friend is a car seat fitter. The base of ours moves around but when the tether straps are on it doesn't. If it is a capsule the base moving isn't a problem because when the capsule is put on and tethered it doesn't move any more. Double check that the car seatbelt is as tight as it will go and it should be fine. Generally when it comes to these sorts of things I do it myself or get a pro to look because otherwise DH and I fight.