In London area (central), it was mainly based on a rent a room charge- either £15/hour; or charge by half day (4 hours) or full day (8 hours). Make sure that it is chargeable whether or not she has bookings- has to be written in the contract. Also, the days have to be set ie she cant change and swap around as she pleases as it will affect you renting it out to others eventually eg, what if a therapist wanted it for the same time every week as well? I had a similar situation when very new and the doc decided to take the mickey and didn't want to pay when she realised that its not so easy getting people in. I'm a doctor myself so also talking from personal experience.
Busy places tend to do a profit share which works well for both parties ie usually 70/30 for the doc. You should also come up with a scheme where if you introduce a client to her, you should get the benefit the first time only ie maybe 10% off for the client if they provide your leaflet/promotion and you maybe get 10% treatment cost as its a marketing cost for the doc. This could work well for both.
