As this is your first post, welcome to the Pure Planet community! When you post here you are not talking directly to Pure Planet but to your fellow customers - and they are a helpful bunch...
First, be assured you won't have to pay for energy you haven't used.
There's two vital questions: was Pure Planet already supplying the property when you moved in? Have you joined Pure Planet yourself?
If you haven't joined Pure Planet yourself, do it straight away. In effect you need to switch ownership of the energy account to you. Use the current ready, but also keep a note of the readings when you moved in. The person moving out should have sent their final readings.
When you have joined, or if you already have, then it's probably also worth contacting Pure Planet. Open the app, click on the three little bars top right hand corner, select Help, choose Wattbot, put 'message the team' (without the quote marks) in the question box and then explain the situation. They promise to get back to you within one or two working days, and they will be able to sort things out.
If you have any further questions, or I have misunderstood your question, post back here and someone will help you further.