"I have seen boilers that have had em fitted, never seen any difference in the kettle"
Scale inhibitors don't
stop scale forming, they just cause it to form a different crystaline structure ... one that is much less likely to stick to pipework. So you will still see it in the kettle, especially if you don't completely empty/fill each time.
Since fitting,my kettle scales up just as much,but the scale doesn't seem to stick to the bottom as much,so gets washed away.Is this what's supposed to happen,or should there be no scale at all?
Yep, that's exactly what should happen with those inline scale inhibitors. They don't remove existing limescale, or stop it forming, because they don't chemically alter the water (like a softener).
"Personally I cant see them working, what happens to anything these magnetic types catch or filter out?"
Limescale isn't iron based, so isn't magnetic, so won't be 'caught' by the magnetic kinds of inhibitor. Again, the magnetic ones claim they change the crystaline structure (ions *are* susceptible to magnetic fields) and are just an inhibitor - not a filter or a cure.
people have told they have to be changed on a regular basis (on the paper work, it says a life time, what ever that means)
I guess, as always, follow the manufacturers instructions
Some say they need no maintenance, some say they have a lifetime of 5 years. On non-magnetic ones, lifetime will often depend on the amount of zinc in the inhibitor, for example, and how hard your water is. The magnetic ones will probably last forever though as the magnets aren't being 'used up' by creating the field.
What happens to any debris these things are supposed to remove ?
As you'll know now, they don't remove debris ... it isn't a filter.
And as for my personal view, yes - I believe they work. I live in a very hard-water area (East Anglia) and have seen scale build up on cold-water float valves in cisterns and F&E tanks. There is a tide-mark on the ball. Recently I went to a job involving a faulty float valve and thought that valve/ball must have only been a few years old because it was (visually at least) in very good condition. It turns out it was 15-20 years old, as old as the house, but then I noticed a scale inhibitor on the main cold feed which was also installed at the time the house was built. All other things being equal, I'd have expected this float valve to have lots of build up on it, and the ball to have a ring of scale around it. Anecdotal, yes.