Hi, just after some clarification.
I've got a no-o retirement visa single entry which is valid till 28/9 can I enter Thailand around 26/9 & get the full 90 days + can you use a re-entry permit in the 90 days & can you get a 60-day extension at immigration after the 90 days.
TLDR : Answer Summary
You can enter Thailand on your no-o retirement visa before it expires, and you will receive a 90-day stay stamp upon entry. You can also obtain a re-entry permit within those 90 days. However, you cannot get a 60-day extension unless you are married to a Thai national or have a Thai child. The only extension available for this visa is a one-year extension based on financial criteria.
90 DAY REPORTING RESOURCES / SERVICES
- Use the trusted Thailand 90 Day Reporting Service to get your in-person report done and mailed to you for as low as 375 THB (even if the online system doesn't work for you).
 - For immediate assistance, contact Thai Visa Centre directly via LINE at @ThaiVisaCentre or Email them.
 - Join the Thai Visa Advice Facebook Group to ask your questions, and get advice from others.