Hi and welcome to the forum
What year is your car? You may well have Isofix fittings in which case it is worth buying an Isofix seat as they are much more secure. Also, use the search facility on here for "rear facing child seats" as there were some interesting and relevant posts.
2005 Berlingo Multispace 1.6i Desire (Iron Grey)
If everything seems to be going well, you have obviously overlooked something.
Any reputable place will let you try it and check that you've fitted it safely. If they won't let you, go somewhere else.
Things to watch are where the belt clips in - you need to make sure that's not potentially going to get bent. When we had seats, we found the Britax First (Club now?) Class fitted pretty much any car, which was helpful as we had a jag and a metro at the time.
If its one of those car seats you can carry, you need to make sure the belt is long enough to get round. In the metro, only the front passenger seat, shoved all the way back had a long enough belt. Which was a bit of a pain as it turned it into a 3 seater.
The Bus: 2009 MkIII VTR HDi 90, 7 Seater