I've had this game for ages but after reading a couple of reviews am wondering whether I could actually cope with it
I think so. It's worth a try.
I think I can manage the mouse/keyboard mix of controls but ....
Can I save whenever I want to ??
Not really. The game keeps track of what you've done, but it will not save your location. Once you get past the introductory part and get to your Relto, the game always starts in your Relto. You can touch the Journey Cloths you find around the Ages and they act sort of like save points. After starting in your Relto, you can link to the last Journey Cloth you touched.
Is there a great deal of jumping and stuff ??
Not really. The required jumps aren't really all that hard as jumps go, and there certainly aren't nearly as many of them as in games like Tomb Raider. But there is a long wait if you miss your jump and fall in a pit. You have to wait for your Relto Age to load, then link back to the Age where you missed your jump, then wait for that Age to load.
Is there much timed stuff ??
There are puzzles that require timing. There is also a bucket puzzle that requires you to run to it.
No. The only thing that can kill you is if you fall in a pit, and when you do that your character always links back to Relto. Momentum is not conserved, so you don't splat. So you don't really die. But in a sense it's like dying because you have to start from your last save point.
Definitely try the game if your computer can handle it. But if your processor is slow, you'll be looking at some long load times.