I have to leave Thailand for at least 6 months after getting my extension. I assume that the re entry permit lasts as long as the extension, but can anyone confirm that for me please?

Correct

The re-entry permit is valid until your extension ends.

Thank you.

As said before, correct. To confirm take a look at the re-entry permit itself. It should say "Valid Until" a certain date. And "Length of Stay Until" a certain date. In the case of my latest multiple re-entry permit these are the same date, which is also the same as the date my retirement extension ends ("Extension of stay permitted up to" date).

Thanks for replies.