Hi Sarah,
WW1 records are now in the public domain., see national Archives for starting point:
http://www.nationalarchives.gov.uk/m...mepage=mh-army
Ancestry.co.uk have service records, pensions records and medal rolls databases all for 1914-1920; I note that there are a few Wm Hancocks.
Another possibility is that he was captured & died of wounds received at a later date.
However, if the date (1922) is correct, coupled with fact that he isn't mentioned on the national roll of the dead, I think he would have continued service thus not appear here. You can still request a service record via a form available from UK Veterens website
http://www.veterans-uk.info/service_...e_records.html, you would need proof of next of kin.
Finally, if you know the regiment of the Black Watch you can try and perhaps discover their movements, [type "Great War Forum" into your web browser -very good forum in which where you could ask for help!].
As far as I know, there was no action in India or Burma during WW1, although the 2nd btn served in Mesopotamia (Iraq) alongside Indian troops, being amognst the first to reach Baghdad, but incurring heavy losses.
Post-1920, I think Black Watch served in India on the North-West frontier, but can't be 100%.
Best,
Riddler