Google Services is killing my battery by keeping phone awake
After trawling through all manner of options I isolated the root cause of this issue. 

Within Google Account Sync I had a look at all available settings and the one that seemed to be causing an issue was 'Sync Internet' option which was just constantly running (and appearing to be keeping the phone awake. 

Switching this off has got back my battery back to normal.

I still have no idea why my phone was trying to sync the entire internet :-)