If he was an AB/stoker called Knocker White, you're fresh out. If he had an unusual name or he is memorable for any other reason (e.g. heroism, terrorism or alcoholism) you may get lucky. Give us his name and where he lived; you never know, somebody might be able to help.
His name was Leslie Grainger from Southport and thats all I know for now, I am going up there on the weekend to try find out more information from his brothers wife ( the only living relation left) but I'm not sure if they know more then I do.
If you are still wanting to find out about your Grandfather there are still plenty of sources of information but you need to be prepared to do a bit of detective work. Fortunately a lot of can be done on-line. Most of the larger public libraries have a family history section that can get you started by finding the records for Births of key individuals like your great uncle that you said had recently passed away. This gives you the names and addresses of their parents and their movements can be tracked through the Census records, street directories and the electoral register. Parish records for christenings also help. As I understand it, once you have the name and date of birth of your Grandfather you should be able to ask about seeing his service records. By any chance are there any of his medals still around - they should have a Service number on them?