Previously always used isofix car seats for my now 15 month old in my car. However we've been given a second hand one for my dh car that you strap a seatbelt around. We have followed the instructions and have it rear facing but it seems so wobbly. The belt is tight but it doesn't seem to hold the seat where it should. Is this just how these car seats are? Having being used to isofix bases we are used to them not moving at all.
Is this safe to use? Baby is over 22lb now so may weight it down by being sat in it but I don't want to take any risks.