I have been sent this which confirms that he was indeed cashiered for bigamy:
Full Name :Gardner, A H A
Incident Date :4 May 1943
Rank Name :War Substantive Lieutenant
Rank (2nd) Temporary Captain
Additional Information:-
An unusual case, plea of guilty, legally in order. This statement was not signed.
Date of Court Martial 3 Jan 1944
Nature of Charge :-
The accused was charged under Section 41 of the 1881 Army Act. This section of the act specifies the "offences punishable by ordinary law of England which may be awarded for the most serious offences, treason, murder, manslaughter, treason-felony and rape". Bigamy
Remitted Or Commuted :
6 months imprisonment with hard labour remitted on confirmation
Sentence :
1 years imprisonment with hard labour and cashiered
Where Held : In the field
Primary Unit :Cameronians (Scottish Rifles)
Archive Reference :WO86/118