Did you mean "service record" or "medal index card"? I will assume the latter for purpose my reply.
RND [E] 1323 = Royal Naval Division [Engineers] Service No 1323 [see below]
RE/101B117 25084 = Medal Roll reference - Royal Engineers/101B117 25084
I guess that will be for British War & Victory Medals.
The Royal Naval [63rd] Division consisted of Naval personel [mainly reserves] and Royal Marines who fought ashore as infantry, there were three Field Companies and and one Signal Company Royal Engineers attached to it, see http://www.1914-1918.net/63div.htm
The medal card is only for British Medals and doesn't always mention Gallantry Awards and even then is a simple abbreviation, so Croix de Guerre won't be on the card. There are separate medal cards & rolls for gallantry medals but they are not online.
The only reference in the National Archives catalogue with the words "Croix de guerre" in the title that fits with the 1918 date [most are WWII dates] is in an Admiralty record:
ADM 116/1576 - Proposed awards of Croix de Guerre and Italian Silver and Bronze medals for valour, 1917 . I suppose he could have qualified for it whilst serving with / attached to the Royal Naval Division.
A general web search highlighted that the item of tracing British recipients of the Croix de Guerre has been covered on the Great War Forum and the British Medal Forum. It appears records are very few a far between and your lucky that your found mention in London Gazette as few were mentioned.
Other than references to "possibly some dusty old file in France", the only other mention of note was "you could look at the Exchange of Medals WO388 Honours & Awards Branch: Exchange of Army Decorations between britain & the allies Registers. At the NA"
This is a complete post by "themonsstar" from the Great War forum on 21 Nov 2006:
"The exchange of honours, decorations and medals between the victorious allies during and after the war was extensive. The series consists of 29 books into which have been pasted the running records of British awards to personnel of allied armies and allied awards to personnel of the British and dominion armies.
All awards were published in the LG and it is from these that the pasted entries are cut. The announcements are from 'advance copies' of the Gazette and contain some info on units which was removed for security reasons before publication. Manuscript annotations record disposal of insignia. The series is arranged chronologically by receiving or awarding country, as appropriate.
You can go to the NA and look at them or you can take Marks, advice and go looking for some dusty files in France or Belguim."
So it would appear you already have as much as is in the exchange of honours etc., in WO388.
From Wikipedia - "The Croix de guerre may either be bestowed as an individual medal or as a unit award. The Croix de guerre medal is awarded to those individuals who distinguish themselves by acts of heroism involving combat with enemy forces. The medal is also awarded to those who have been "mentioned in despatches", meaning a heroic deed was performed meriting a citation from an individual's headquarters unit."
It is likely his service record would have a note of the award or mention of some related award, but that assumes his was one of the 30% or so records that survived the WWII bombing or was reconstructed from pension records. If you can confirm the Unit he was serving with at the time you may be able to trace something via War Diaries. The only other possible source could be the Royal Engineers Museum.
= = =
I prepared the bulk of the above yesterday and reading through it before posting just now, I noticed one of the criteria for an award of The Croix de guerre was "also awarded to those who have been mentioned in despatches ". I rechecked the NA's catalogue and although I have no knowledge of the content I found this that may be of use:
WO 32/4296 DECORATIONS AND MEDALS: General (Code 50(A)): Certificates for Officers and men of the armed forces who have been mentioned in despatches, 1917 - 1922