Gerard
My paternal grandfather fought in WWI and was taken prisoner of war. My father found out from the MOD that his records were destroyed during the blitz but I did write to the Regiment he served in and they found the records when he re-enlisted as there was no work at the end of the War. For 8 pounds (sorry so sign on my keyboard) they will copy and post them to me.
My maternal grandfather fought in WWII and I cannot find his records on ancestry either. I have written to the RASC and hope they have something. He is not listed on ancestry as a soldier who died in WWII I think that maybe because he was sent back to the UK and died there rather than overseas.
Apparently Regiments keep War Diaries and this may help you.
Good Luck.
Susan