Oxfordshire/Berkshire. Do you stand your boy at stud? Any chance of pictures?
I've got an (also unregistered!) appaloosa/spotted mare and have been thinking quite a bit about breeding a mule recently. Life is short and mules are cool and I think a Grand Noir du Berry is most likely to "compliment" my mare's conformation, moreso than a Poitou, which seems to be the most common "mammoth" donkey in the UK; I've wanted a mule for years to ride and drive, but as I am quite tall, need one at least 14.2hh, and those don't seem to be so common at all, sadly. So breeding seems a good option; the mare has correct conformation and movement, a superb temperament and was a broodmare in her last home, being proven to be an excellent mother. Happy to send photos of her.
I'm not definite of doing this yet (and it wouldn't be until spring 2019 anyway), just putting feelers out.