Well, sirDavid, I have to confess instead of trying to solve your problem, I immediately tested it for alternate solutions. You'll be happy to know it came up clean.
Just one thing, the 2 pawns could be replaced by a single Black pawn on g5.
Interestingly, Replacing with a white pawn on g5 gives a different solution:
Just messing around with the Nalimov tables, you occasionally run into neat mates. For example, here's a mate in four, with a small edit to make the solution unique
. Hopefully, my change doesn't add a different solution (I don't believe it does).