Hi Lee,
The Regimental War Diaries of the 12th Londons are available for inspection at the National Archives in Kew. The catalogue reference is WO 95/128. These diaries cover the period Dec 1914 - Jan 1916.
Be warned however, that whilst some War Diaries are very complete and contain extraordinary details, some of them can be very sketchy. Sometimes casualties are mentioned by name, but usually only officers merit a mention, so don't expect too much. Nevertheless, it's well worth trying, and you may hit gold. Hope that helps!
Good hunting,
Swampy