Filled in online spouse application and paid fee’s. When must I book appointment?

I have filled in the online spouse visa application and also paid the fees. I have now reached the page where it says I must book an appointment at the UKVCAS office before 27th October. However my wife’s fiancé visa runs out on 14th September must the appointment be booked before the 14th September?

I understand she can stay in the UK while awaiting a decision, but at what point do they consider you have officially applied? When you have paid the fees (today) or after you have attended the appointment to carry out biometrics and given them the required documents? I don’t want to book an appointment for 20th October and find her right to stay in the country expired on 14th September.

Normally, once you have 'submitted' the online application (by hitting 'Send', or whatever it says) you are considered as having applied - which must be done ON or BEFORE the Expiry Date of the current Visa/BRP! - so as long as you upload any document evidence required within the given time frame you should be OK.
Obviously the computer system isn't concerned that the current one expires on September 14, so that suggests you should be OK.

Once you’ve applied ie hit the submit button you have applied! Your immigration status remains the same until you get a decision you are on a fiancée visa so those rules apply status applies.
Memory is a bit foggy now with the passage of time but it used to be
Once you hit the apply button the next screen was additional actions and this is where you book the bio appointment in your case and depending on the biometric status of your passport you may not need to do an appointment the system I think tells you this