After installing 27.09 on forerunner 965 the battery is draining at very fast rate

Have you done a restart since the update?

HI, I have a FR 165 and also see this behaviour with 27.09. Additionaly my heart rate detecting and all related functions like sleep detection and body battery are not working anymore. Seems to be the worst release Garmin ever did.

10th restart was a charm. All back to where they should be. 

Except if one extra restart (after all the upgrade itself focus a restart) is not enough then 9 other restarts probably didn't fix it either. So either something else you did/happened fixed it, or it's not really fixed, and you'll see it happening again after a while.

same issue on my FR955

Having fierce issues since this most recent update. Watch keeps crashing. Garmin support said the only option is for me to pay €184 for a replacement device as my warranty is up but the watch has been perfect up to this last week 

That's very disappointing customer support. And there's a chance the replacement would have the same issues too due to the firmware. I suggest waiting it out, there have been many complaints and hopefully they'll release a fix.

I turned off auto updates, will be staying on 26.08 for a while.

I’m going to see if I can download the previous update and if that will make a difference to the watch 

You won't find the previous one online. Garmin should ideally provide a way to roll back.

If you go to the system menu on the watch, then about and push the light button 8 times there might be a developer menu and option to reinstall software. I think it's worth a try.