While I would do this, what about the next house owner who might not.

Do you care? If so, leave instructions. If they're too scared to do it
themselves, they can always call a plumber or handyman to do it for them.

How often will it need topping up ?

I have not topped up the Pandora since it was installed in October. That's
fractionally shy of six months. It still works perfectly. The flow rate
compares well with my mother's Megaflo and is determined almost solely by
the mains supply. At some point, I'll cool it off and see how much the water
level has dropped. Given the design, I suspect it won't have dropped at all.

The design has what is referred to as an "anti-evaporation" bubble. This
expands when the cylinder is hot and looks pretty impervious to water
vapour. I doubt much water could escape the system if it tried. The design
is certainly not having the hot water sitting in a puddle at the top, just
asking to be evaporated, like you would find in a ball valve system (where
it wouldn't matter, of course).

Christian.