The success or failure of this tangy drink is partly reliant on the quality of marmalade used. For ease of use, choose "fine cut" or even "no peel"/"shredless"...
For me the marmalade simply refused to dissolve in the gin. So I put orange juice in a pot with some brown sugar and reduced it until fairly thick, but not too thick to mix into the drink. Worked very well, but I’ve never had a decent version of the marmalade version to compare!