UK Landlord Forum / Life

I am in the process of finding a lodger, facilities will be shared such as kitchen/living room and the house is my main residence. I have a lodgers agreement that ensures I do not give the lodger exclusive occupation. I have a room that is currently vacant and I have found someone I believe will be suitable, she is female, and asked if she could put a night latch on the door, which although I see as completely understandable, I am worried as to whether this would then change the legal status of the agreement into a standard tenancy, which I do not want, or have permission from insurance etc to do. Most of the information I have found suggests that locks are a big no no for this reason, however some information suggests that as long as it wasn't a standard key that could lock the room when she goes out then this wouldn't be an issue as she still wouldn't have exclusive occupation. Effectively I am fine with her being able to lock the door whilst she's in the room providing it doesn't alter her legal status/rights to occupation.
Any advice would be much appreciated!

It's a bit of a grey area.
I'd find a different tenant.

Sounds like she's scared of you 
If locking it only from the inside, a bolt would do this wouldn't it ?
Not sure what that approach has regards insurance/legally.

Maybe this is the thin edge of the wedge of requests.