Hi Acorn and welcome to RootsChat !

I can't tell you specifically about your Grandfather but until the experts get here ... I can give you a little background !
The navy differed from the army in that all men who served got at least the British War Medal even if he was on home service or joined and discharged after a short time. The Navy rolls are part of the ADM171 files at the National Archives - they are not on a card index. The total entitlement is entered on a roll and there is only one roll for all medals. There are no records of issue of for the Mercantile marine medal - and also there are no records for the Silver War Badge.
I believe your Grandfather was involved in the last act of the Somme offensive took place in the Ancre sector from 13 to 19 November 1916. The operation went ahead after repeated postponements - largely because it was hoped that a late British success might create a favourable impression at the inter - Allied conference at Chantilly on 15 November.
In dreadful conditions the Fifth Army ( as Gough's Reserve Army was now called ) attacked astride the River Ancre north of Thiepval, to reduce the German salient between Serre and the Albert-Bapaume road. The 51st (Highland) Division took Beaumont-Hamel and the 63rd (Royal Naval) Division entered Beaucourt but the village of Serre stayed in German hands.
You may know all this already ..... but thought you maybe interested ........
http://www.webmatters.net/france/ww1_ancre_2.htmAnnie
