Got back from the vet about two hours ago and I'm feeling a bit more positive and reassured, although it's still a waiting game. William was bright as a button by the time we set off so it seems that he was only quiet when he was actually being sick and probably because I hadn't fed him.
Our lovely vet Claire gave him a thorough examination, including having a good feel of his abdomen to see if she could feel the stone. She couldn't, and although there are some areas that can't be checked that way because they're out of reach, she said that William was so relaxed that it made it easy for her to check the areas she could reach and was a good indication that he isn't in any discomfort.
She thinks that it's entirely possible that he's already got rid of the stone, even though I've been vigilant and haven't seen it. If he hasn't, there's still a good chance that it's slowly making it's way through William's system and could take up to 5 or 6 days to pass through.
The vomiting could be coincidental (William does vomit every so often for no obvious reason), or could be because the stone may have scratched his insides on it's way through, or it may be causing a partial obstruction. As he only vomited small amounts and is well in himself she is treating the symptoms at the moment, so gave him an antibiotic injection and I have to feed him chicken and rice for a few days, starting this evening. The duty vet will phone me tomorrow for a chat and if William becomes unwell at any time I will contact the vet immediately.
X-ray and surgery are still possibilities if he actually becomes ill, but Claire says they're certainly not indicated at the moment and is hopeful that he has, or will, pass the stone naturally.
Apparently if he'd been taken to the vet immediately after swallowing the stone they could have given him an injection to make him vomit. It needs to be done within about 30 minutes and obviously only where the object is small enough to be vomited up. If only I'd known!
William has been out for a walk in the New Forest with his Retriever friend Ruby since the vet trip and was his usual happy, bright self. He did one poo, but no peach stone. My friend Maria and I got some very strange looks from other people as we dissected the poo with a stick!
I'm now hoping to relax for a while and catch up on some sleep, cuddled up with William. I'll update on COl when anything happens (hopefully a peach stone poo)! Thanks to everyone for your support and kind thoughts.