Hi,
Your solicitor can answer this, and all the questions you've asked today. You're paying them a fee.
Apportionment is simply a way to split annual charges fairly between seller & buyer. If the seller has paid LPT for the year, and you get the property at end of March, you refund the seller 9/12 if the annual charge.
Because they've paid the charge for the full year.
You're moving in during the year, so part of the charge relates to a period when you own the property.
Apportionment is part of a standard law society sale agreement