Hello! Just wanted to pop in and say that I've started a new world, and am not having further problems. Aside from having updated Java, I didn't do anything differently...so either Java was it, or a recent update fixed it.
Thanks to everyone for your help and advice, and to our modpack maker/s for their fine work on this pack.
Really liking that the new versions of Minecraft are so fast to get some interesting mods, and that there's folks taking the time and effort to get them together in easy to use packages like this.