Bathroom Door Lock

Good evening,

My first post so I hope someone can help. We've recently got rid of our Series 5 Senator Virgina and purchased an Olympus 525 (change of family circumstances). It's a nice van although obviously not with the plushness of the Senator.

Anyhow - we have a little problem in that our 23 month old boy keeps going into the bathroom and turning on the tap. Naturally we want to keep him out of the bathroom. On the Senator there was a lockable door which we could lock from the outside (with a coin). The Olympus has no such lock.

Has anyone adapted their van with a lockable door handle? Anyone help me source a part to do this?

Many thanks

Gareth