Hi all, Casper here.
I was back at the clinic on Sunday for a recheck. This time mum was able to get an appointment with the vet that she openly adores who was out of town when we went in before. He said it was definitely chin acne and not rodent ulcers or some sinister fungus. What a relief! He also said sometimes it can take a couple of months for the chin acne to clear up and we should keep bathing the area with the Chlorhexidene anti-bacterial soap daily. Luckily I don't mind having my chin washed, I just lie on my back in dad's arms with my head hanging upside down and mum cleans me and tells me I am a very good boy!!!
Thank you so much to everyone who left a comment last week and suggested treatments, it was very helpful :-)