Water fill cap

I’ve recently purchased a Harmony (Peugeot boxer van) 1995 model. Having a play this weekend we realise that we don’t have a key for filling the fresh water and therefore cannot remove the filler cap. Any advice please?

First thing I would do is contact the previous owner and ask if they know where it is.

Next thing to check is: Is it actually locked? Some can be a bit stiff to open when they are unlocked. If it is, you probably don't really need a key. Doubtless there have been horror stories in the past of people putting unwanted things in people's water supplies but... I doubt it ever actually happens.

If you can't locate a key then you will probably have to remove the old one and replace it. A sliding hammer with screw-in point will typically remove the old one, but be warned - the chances of a new one matching the space where the old one was fitted are not that great. We had to adapt and improvise on our 2006 Trooper because water inlets of the size fitted at the factory are no longer available...

Have you actually tried all the keys that came with the vehicle?  

On ours, we have a key that fits the cab doors, petrol filler cap and the ignition plus a separate key which operates the locks on the hab door, gas locker door, toilet cassette locker door and - the locking water filler cap.

Worth noting that when the cap is locked, it turns freely and does not engage with its clips.
So when you say you cannot remove the cap, is it because it turns freely or that (as Roopert says) they can be stiff.
If it is stiff, push in as you turn.
If it turns freely, then it is possible to drill a hole and insert a screw to lock the 2 parts together (as the key does when unlocked)- I can advise