I have been attempting to go down the diving route
for a refill.

It seems that our aviation cylinders require hydrostatic
testing every 5 years (every 2.5 years for subaqua
cylinders). I think the European standard is CE as
against US DOT.

Having tested the cylinders, they may want to 'O2 clean'
the cylinder before filling it with pure Oxygen. Because
there seems to be a worry that any stray hydrocarbons
will set off an explosion.

The diving shop said that they would replace all the
O rings on cleaning. DG dont have any o ring sets.
So I am currently wondering what to do if I reach that
stage.

The fitting is likely to be totally different and you
will need the right adapter. The DIN fitting on DG
aviation cylinders is an external DIN and totally different
from the internal DIN on diving cylinders. Diving cylinders
also use 'yoke' and medical cylinders use 'pin/index'.
RD aviation sold me an adapter (at a price).

I am not sure how easy it is to get Oxygen at 200 bar
from diving shops, because most is mixed with air as
nitox or trimix. Thus they can start with a relatively
low oxygen pressure (50-100 bar) and then top it up
to 200 bar with breathing air.

So I still have a comparatively new cylinder with 100
bar of oxygen and I shall have a few more flights before
I get this out of date cylinder hydrostatically tested
and refilled.

Rory