I would suggest that you contact Scottish Power and the Extra Energy administrators to find out the agreed handover figures. If these are obtained contact Extra Energy administrators and ask for a final bill based on these figures (there is of course a risk it might result in either a credit or debit bill being issued). The administrators number I last used was 0800 368 5452.
You will need your account numbers for both your EE and SP accounts to make this easier.
I'd recommend that whatever you communicate to either party this is done in writing (email, chat, etc) you keep a copy or if contact is made verbally then you record the conversation so that you have a record to fall back on if needed.