New Android 4.1.2 Jelly Bean firmware is now rolling out to Xperia J (ST26i) handsets as expected. Firmware version 11.2.A.0.31 offers no noticeable changes over the previous build (11.2.A.0.21) so is likely to contain minor bug fixes. If you have updated, let us know of any improvements.
Thanks Aaron and Anurag!