Welcome kbendi! I do not see a serial number on the movement, just "Dent, London". The number on the case usually has nothing to do with the watch. It is a number from the case maker. In most instances, movements from one maker were put into cases from a different maker, either originally or at a later date. Go to this site and read all about Dent...
http://www.dentlondon.com/about/history.phpValues on antique pocket watches are based on quality of the movement, how complex it was, whether it runs or not, condition, original face, etc. Cases are valued by gold content, age, decoration, condition, etc.
Being that yours is from your great grandfather, it would also have an immense sentimental value that cannot be put into $$$. Treasure it.
To see Dent watches from sellers, just put "Dent London pocket watch" into a search engine and look at what comes up.