Thanks everyone for your advice and suggestions. I'm leaning towards the Casio PX-S3100 over the Yamaha CK88. The Yamaha is about twice the price, and factoring in a soft case and music rest it is quite a lot more. From YouTube videos I also think the organ sound on the CK88, while perfectly good, is not as good as Hammond B-3X on my iPad. So I can use that (with a small 9 fader Midi controller for the drawbars), and rely on the inbuilt organ sounds of the Casio should something go wrong mid-gig with the iPad. But this has never happened before so hopefully an insurance policy I won't need. The other sounds of the Casio (pianos, EPs, strings, synths) all seem very good quality. Both keybeds seem fine and probably pretty similar (lightish hammer action), although unfortunately I can't try them out near where I live.
On the other hand, the CK88 has lots of features e.g the quick layer and split switching and ability to tweak settings on the fly. I'm not a very experienced live musician so I've never tended to use stuff like that much, and tend to have presets ready to go rather than adjusting in the moment. But are these features that would be very helpful to have as I play more stuff live? I may be missing some other features that really set the CK apart and justify the extra cost. I'm also slightly concerned the Casio may be a bit fiddly to switch sounds too via the touchscreen, although I plan to use an iOS app called KeyStage to control this primarily. I mainly play at functions and weddings, pop and rock and some blues / funk stuff. All thoughts gratefully recieved!
Cheers,
Adam