Citroen Dispatch 2003 1.9 back door problems

I recently bought a Citroen Dispatch from a private seller for my first van. It drives absolutely fine but neither of my side doors open, they do from the inside but not the outside, how can I fix this?

More importantly, the back doors lock but as I'm driving they pop open, I can't seem to notice what causes it and as they're locked they can't fully open but this can't keep happening. It must be the left hand door but if I close it and give it a pull nothing happens, I've shook it a bit and given a good tug but the latches hold, I can't see why they don't when I'm driving.
Nothing in the back is pressing on them and nothing slides and hits them.
Is it a simple fix? Replace all the locking mechanism for all the rear doors?

Thanks

Carl