The vet had a good check of Charlie's glands but found nothing sinister. He also gave his tummy a good feel to see if something was hanging around and having difficulty getting passed. He suspected Charlie had maybe eaten a bit of string or something and was having trouble passing it.
However...Charlie has again been darting in and out of his tray. I inspected after he peed this morning and found blood in his urine. I was so alarmed and rang the vet who suspects a urinary tract infection so am heading back there at 11am. Am so, so worried about little Charlie. He seems fine - he's eating, playing, being mischievous as usual but clearly something is very wrong.
I'll let you all know what happens from here.