The letter "M" would be the date mark on the teapot. In order to determine the date, you will have to look up the mark on the chart for London date marks at the Silver Encyclopedia Hallmark website. The date mark changes all the time, so there is no way for me to tell you what it is without seeing it. As for the value, the best way to see what people are currently paying for similar items is to go to eBay. Do a search for "small silver teapot" (if you include the maker's name you can narrow it down more for better results). Search the "completed" listings to see the actual price that items are selling for. Sorry I can't help more.