Expansion vessel mounting

A mate of mine was proudly showing me his brand-new house.
The plumber had installed an Ariston sealed hot water system. He has mounted the safety valve in the pipework at the back of a cupboard above the cylinder (fair enough), but physically screwed an 18L expansion vessel horizontally into the valve without any other means of support. Surely this cannot be correct (or safe)?
Firstly, it's not easy to get to the safety valve to test it.
Secondly, the whole weight of the expansion vessel (and its contents) is being supported only by the valve/pipework.
And thirdly, do these things function correctly if mounted horizontally?

I checked the installation instructions for my own system, and whilst all the pictures show vertical mounting, there are no actual instructions to say "must be mounted vertically".

It all really depends on the size of the vessel. for smaller domestic one I always screw them into a backplate elbow which can be fixed to the wall.

I assume the guy that installed it was a plumber and had the ticket for un-vented

When the gas depletes, that's going to weight 18kg!
They work any way round
EV's are usually mounted on wall brackets!