I created a new workspace in order to produce a project based on the ImageGallery project.
I slightly modified ImageGallery.src and saved it as Photo Library.src. When I compiled it the splitter control wasn't there.
After a lot of investigation, including modifying the ImageGaller.src file in my workspace to be identical with the Photo Library.src file (the splitter control was ok in this), I tried renaming Photo Library.src as PhotoLibrary.src. This solved the problem.
Why did the space in the filename stop the splitter from working?