The National Archives are digitising their collection of WW1 Unit Diaries and are providing access to them on their Discovery Online Catalogue:

Simply enter a Regiment, battalion, Brigade or Division number into the search field provided on the above linked page and the Discovery System will take care of everything for you and pull out all the diaries currently digitised that match your search terms.

The National Archives have currently got around 1.5 Million Pages completed so far with many more to be released throughout the year in commemoration of the First World War Centenary.

They have also launched "Operation War Diary" in an effort to Tag their diaries with appropriate terms to make them easier to search and they want YOU to help!

Follow this link if you are interested: https://www.operationwardiary.org/