As others have said, that's a wonderful watch even without considering its sentimental value. It would probably fetch over £2000 if sold. Here's a
dealer that specializes in this type of watch, and you can see that the prices listed for your model range from £3,295 to "price available on request."
When you ask if the watch is worth "restoring," it's probably good to be careful with the terminology. "Restoring" has a connotation of making the watch as good as new, and might include refinishing the dial, polishing the case, etc. You almost certainly do not want to do that. It will significantly reduce the watch's value (watch collectors place a great premium on originality), and, on a personal level, diminish the connection to your grandfather. It is after all, the nicks, scratches, and blemishes that made the watch uniquely his.
"Servicing," however, would definitely be fine. So long as the service was limited to disassembling the movement and cleaning it, and then reassembling it and re-lubricating it. If necessary, some internal parts (e.g. the mainspring) might need replacement, but if the watch was running fine and simply stopped one day (i.e. it wasn't dropped on a hard surface), that will likely be all that's required to get it running as good as new. I don't know about U.K. prices, but in the U.S. that service would cost around $300 from a competent watchmaker. If you do have the watch serviced, finding a watchmaker that specializes in vintage timepieces would be well worth the effort. I'm sure that there are forum members here that could make recommendations if you don't have someone in mind already.
If the watch's value is a significant surprise to you, enough to make you worried about wearing it, then there's no need to rush on the service. It won't hurt the watch to sit idle for a while. The biggest danger in delay is that parts are getting scarcer as time goes by. If you do plan to continue wearing the watch (I would, fwiw), one thing to be aware of is that the watch case is, I believe, alloy rather than steel. (I can't tell for sure from the photos.) That alloy becomes somewhat brittle with age, so there is a danger that one of the lugs could break off. I wouldn't describe it as fragile, but it's something worth taking care with.
Thank you for sharing the watch with us here. It's a very refreshing change from more typical queries.
