Guys,
Finally i managed to solve the riddle. Apple came back to me again stating that the battery was normal which got me thinking if this was a O/S issue. I kept testing the device and came across the Battery usage tests (Settings > General > Usage > Battery usage) to display Usage | Standby which were showing close values. I tested the phone with killing apps to see if it was any app which could not let the phone sleep.
Amazingly, i found that when on Wifi, the values remained close indicating that it was something over Wifi which is not letting the phone sleep.
Thereafter i tested the phone with my Mac over an application called Activity Monitor to display the processes which start when i start wifi. There were 2 processes which were invoked - Bird and CloudD. Narrowing this down brought it to be processes liked to iCloud Drive.
As soon as i disabled Cloud Drive ( iCloud > iCloud Drive), my phone was back to great battery performance.
I am yet to figure out more about this problem but this fixed the phone battery draining issue over Wifi.
Hope this helps !
Gary