Help with Royal Engineers Units [Edited]

[richredwood]

I'm wondering if anyone could help at all with establishing in which RE unit Henry Augustus Allen, born 1886 in Croydon, might have served during WW1.
He volunteered for the RE in Jan 1915 signing up as a sapper in Norwood, South London, for the duration of the war [Surrey Recruitment Registers]. He first entered a theatre of war [France] on 1 Sep 1915.
He received the Victory, War, and 1914-1915 Star Medals [rolls and medal cards available].
His service number was 63957.
I can find no service record for him and otherwise have no other details or clues about his service.