Hi Shawbags,
You may be having the same problem I had with the first Berlingo I bought, 2004 model.
When you tip the back of the large rear seat section forwards it causes the middle seat belt to lock out as if its had a crash and it won't release the catch to the load deck in the centre of the rear seat. This is because the belt webbing retracts too far back into the reel cassette and causes the inertia mechanism to lock out.
I ended up having to take the seat belt apart, release the inertia mech and put everything back together again numerous times!!!
Citroen know this is an issue and there is a modified belt retraction runner/restrictor that fits on the top of the seat where the belt goes in. If you look at the belt there is a little button on it that is supposed to stop the belt going in too far so you can fold the seat. But the on the early models the plastic guide on the seat top is too wide and the button goes through when you fold the seat! Just go and buy the latest seat belt rear seat belt guide from a 2005 model. It fits straight on and your problem will be solved.
Sorry this post is a bit long winded, it always takes longer to explain than it does to see it in the flesh so to speak.
Hope this helps. :wave: