Whilst these are undoubtably the best source of information, the chances of you finding the man concerned are limited, about 1 in 3 to 1in 4. This is because the vast majority were destroyed by fire in the Blitz. Even the ones that remain are in varying degrees of readabilty due to burning / water damage. These are stored on microfilm at the National Archives in Kew,London. They are also available again on a pay perview basis on the Ancestry website
What about the regimental museum, my Grandad was in the Royal Scots?
A number of things to bear in mind about regimental museums, they do not hold service record, with the exception of the Guards. They tend to be run by volunteers who although generous in donating their time are exceptionally busy people. Access may be a problem as well and an appointment is sometimes neccesary. I would think about going to the regimental museum once you had exhausted other avenues.
The local newspapers of the time.
A goldmine of information on local lads, some even run to photos and extensive stories.These can normally be accessed at your local studies library. The Scotsman cab be accessed online on a pay per day basis or for free for Edinburgh Library members.