I realise that your problem is now solved but wonder (in a positive way) about your earlier investigations, perhaps there is something for the rest of us to learn. Where did you look earlier and how might searches be more successful? So questions aimed only at a positive review of process and I hope that that is fine with all concerned; we can all gain from good processes and guides.
Wright and Round keep a lot of Marches and have three different sets of twenty March books (the Handy, Classic and Famous sets). They might even have other Marches that aren’t in sets and aren’t on their website. I believe that there used to be a National Brass Band archive too (now under the care of BBE?) and some academic institutions hold collections too, maybe they are and there are other sources of music. If a March is associated with a Military Band / Regiment then maybe it or its successors have copies - an IIRC there is still a central school of military music. Of course identifying keepers of old music is one challenge and getting them to help might be another.