Series 3 door comes open when going over bumps

Hi I have removed and refitted the drivers door on my series 3 lwb. I re painted the door bottom and fitted a new door top. Now I have refitted the door it comes open when I go over a bump (even on road use).when I took door off I left hinges fitted to bulkhead . How do I cure problem?

Striker plate probably needs adjusting.

I have not tried adjusting the striker plate yet as I hadn't changed this and hoped that leaving this alone and just bolting door back on would be ok. How do you set striker plate correctly or is it trial and error. Thanks for your reply

If the door still fits on to the door rubbers as it should then it's the striker plate. If its misaligned then it's the hinges.
Just educated guesses until the draught stops blowing your wig into the back.

Check that the plate bolted on the chassis just behind the wheel is tight on chassis and bulkhead,if loose bulkhead can move some and cause door to open.

Are you sure you didn't buy the car from a circus?