Car locks fine. But when you unlock it, it locks itself straight away. Does this with the key fob, with the key in the lock and when unlocking with door pin.

Hi
One of your central locking units is faulty,to find out which one it is you will need to remove all the door cards and unplug them one at a time till it locks and stays locked,sometimes when you have a fault with central locking you can hear the faulty lock operating it will keep clicking

Hope this helps

98% u do not have any faults with ur central locking. If u did then u would be having problems both locking & unlocking ur car.
This is a very common classic saxo problem, as any decent mechanic can tell you.

What has happened to ur car is that the actual handle mechanism is starting to freeze up due to corrosion on its return spring. This causes it not to return its link rod to the locking mechansim to its full stop position, so the lock thinks that someone is trying to open that door handle all the time.
This problem can be fixed with no new parts & without removing any of the door cards. (besides, only an idiot would remove all the door cards just to find the fault!).
The handles can be removed from the outside by drilling out the rivets that hold them, prise them up & out from the car & then unclip the link rod to the lock. Then spray the handle pivot with copious amounts of WD40 & repeatedly open & close it a LOT of times. Eventually it will spring closed by itself when u open it (i know it already does this on the car, but it is not the handle spring but rather the lock spring that is doing this. Once u remove the handle from the car u will see that when opened it does not spring back.) Once the handle can spring closed by itself u should coat the pivot & spring with copper grease & refit with new rivets.
To give u an indication l can remove the rear door handles in 16 seconds & the front ones in just under 1 minute (they r slightly more awkward). The rear handles r usually the first ones to give bother as they r used the least.