... I used to use one of these:
http://www.legendcookshop.co.uk/kitchen-devil-rollsharp-sharpener-24878-0.html
But have since been warned not to due to it causing more damage to the blade than necessary.
Very possibly correct. Some of the devices which ask you to draw the edge through the sharpener can tear the edge quite badly, especially some stainless blades which tend to be more susceptible to that kind of damage.
I'm not confident enough to use a flat stone since I'm not sure I couldn't keep the same angle each time.
Possibly not necessary. If you want a convexed edge, for example. But it's easier than you think, and there are plenty of threads about sharpening on this forum for you to spend some quality time with.
So any suggestions to a sharper that can be use on all edged tools from kitchen knives to garden shears by a complete novice?
Nothing will really do everything although some things will come close. India files for example. The likes of the Sharpmaker and Lansky tools reduce the skill needed (and fun had?) but they tend to be a little inflexible. They won't do garden shears at all. Sharpening shears is
completely different from sharpening knives because they way they cut is completely different from the way a knife cuts. Most people will try to sharpen shears (and scissors, and e.g. tin snips) in completely the wrong way and wreck them. It wouldn't make much sense for example to try to use a strop on a pair of garden shears. I'd happily use only a coarse stone on garden shears, but if you're using a coarse stone on a knife every time you sharpen it, something is probably very wrong.
Consider a steel. Very easy to use, shouldn't take too much metal away so you'll still have some left to practice with. That's all I ever use on some of my blades. Don't try to use one on the shears.
It's worth practicing with simple stones, or something like emery paper on a mouse mat. Cheaper and can do more than the expensive gadgets, but it may be harder work.