After much searching, trial, error and re-trial...below is my suggested solution for those who like me were fed up of having the X2 die on them while sleeping!
Option 1 - Enable Hibernation
This will basically disable Windows 10 "connected standby" function whereby the tablet is supposed to go into a deep "resilence" sleep while somehow still being able to keep checking e-mails and stuff in the background. The problem is that because many drivers (such as the Intel SST Audio Device (WDM)) do not seem to have been implemented well for the x2 on windows 10, then the tablet continues to drain power.
The solution is to go back to using good old "Hibernate" function. It will take a few seconds longer for the x2 to resume back into normal mode and cost about 700mb in HD space which is used by the hyberfil to temporarily store some of the data from RAM memory. The x2 will generally enter hybernation 5-10 mins after sleep, but you can set this below.
To enable hibernation
Go to Control Panel\All Control Panel Items\Power Options\Edit Plan Settings
Open "Change Advanced Power Settings"
Expand the "Sleep" sub-heading and under "on battery" select Hibernate.
Option 2 - Persevere with "Connected Standby" Sleep
Unfortunatly in my experience, this way lies only frustration and dissapointment, but you can try and identify what is stopping your x2 from entering sleep and disabling / upgrading drivers to see if this helps. To help you on this quest you have the following commands which need to be run as administrator.
Press Windows key + X and choose to open command window (as administrator)