I currently own a flat (UK) and have a mortgage for said flat. My girlfriend is looking for a place to stay and it would seem mutually beneficial that she stays with me. She has offered to pay 'rent' money to me, as it would seem to make more sense that I get the money than some random landlord a few streets away.

Neither of us have done this before so we would like to make sure that there are no precautions we need to take. 

Here's roughly the arrangement I've proposed, with numbers nicely rounded:

My current Mortgage costs **£300** a month and my Gas/Electricity/Internet comes to **£100**. Current average rent in the area is **£450** per person a month, so I have offered to let her stay at mine and give me **£200** 'dig' money. This helps me out with my bills and is significantly cheaper than what she would normally pay. Also, the money stays in our couple meaning I have more disposable income which ultimately benefits us both as well. Seems like a win-win.

The question:
Is there anything worrying about this arrangement that puts either of us financially or contractually at risk?

Key concerns:

* Should I be declaring my girlfriend as a renter and thus pay tax on what she pays me?
* Is there any risk of my girlfriend 'owning' part of my property under this proposal leading to a mess in the future if we were to split up?
* Should we enter into a contractual arrangement that we both can sign to cover ourselves from either or both of these issues?

Edit: after a few comments, I thought I'd add some extra info. She currently lives with me rent free and it is her idea to pay me now that her old lease is finished (she was splitting time between my flat and her old flat). I'm not dependent on her money in any way.