Determining whether or not a mark is genuine is always a minefield, and this of course, was the hope and the very intention in the mind of a counterfeit punch maker. I too, am not totally convinced by this set, but if indeed they are spurious, they are a good imitation.
I agree with Phil's thoughts regarding the proportions and the detail of the crown, these are what make the marks themselves suspect. What I would also add is the apparent damage to the side of the Leopard's head punch, which appears to be result of more than one shard flaking off over a period of time and I would have thought the London office would have discarded such a punch long before it got into this state.
As to the maker's mark, it does, as Miles points out, look to be that of Dinah Gamon. This very fact, perhaps, when added to the suspect marks may add some fuel to the fire. The widow Gamon may have entered the mark with the hall to clear her deceased husband's work in progress, or perhaps to keep the business as a going concern, but either way, she may well have had no choice but to put her trust into hands of others. I don't recall seeing her mark that often, so perhaps whatever form her business took, it was maybe short-lived, but whatever the outcome, I wonder what happened to her maker's mark punch? It would have been her property and become just another part of the workshops tools and equipment, and when they all was disposed of, well who knows where it may have ended up.