Hi Clair_UK!
Battery life varies depending on use and other factors. Examples are detailed here:
Apple Watch - Battery - Apple
Apple's tips for maximising battery life and battery lifespan on Apple Watch are detailed here:
Batteries - Maximizing Performance - Apple
If your Apple Watch is experiencing significantly shorter battery life than Apple's examples suggest that you should expect, then it may require service. Before contacting Apple, it may help to follow these steps, which have previously resolved battery life issues for other users, including after software updates, when usage has not otherwise changed (check after each):
Having tried the previous steps, if you are concerned that battery life is shorter than it should be, then I suggest contacting Apple Support (remote diagnostics and/or mail-in service may be available, if required), making a Genius Bar reservation or visiting an Apple Authorised Service Provider for assistance: