you also have to meet the financial requirements for a 1-year extension of stay permit out of an O/A Visa. These are either 800,000 THB on a bank account in your sole name, seasoned for 2 months on the date of your application, or proof of a monthly income of a minimum of 65,000 THB. Seeing you are from the UK, the later option would require a full year bank statement of incoming from abroad 65,000 every month for the recent 12 months (because your embassy does not issue a letter of income proof any more). That and the mandatory Thai Health Insurance showing cover for the whole duration of the year you will receive the extension for, and you will be fine to go (plus passport, TM30 receipt, 90-days report receipt, Bank book, etc)
it looks like you need to do the application for the third extension short BEFORE September 30 deadline, and it will get you 3 more months, taking you to December 30. James Miller posted the picture that interpretes the rules regarding STV. It is all there, you just have to read it
you can do a 90 days report AT ANY IMMIGRATION OFFICE inside Thailand by visiting them personally . It is NOT limited to the branch of Immigration servicing the area where you reside
in case you are running out of time (which I think not as the bank issues a "rab roong thanakan" on the same day) you can still get the "family visit" 60 days of extension
you need to bring a bank letter of guarantee covering the recent two months plus must update your bank book on the day of application, by making one small withdrawal or transfer
James Miller i have non o visa now im looking for extension based on child and wife" . . which means he can get it based on either kid OR based on marriage with wife