I like you have been looking into air set ups and the best advice I have been given is to try different bottles with your marker and see which one feels right .
You should have a 'born date' and 'final date' with 15 years inbetween
The due test dates would be 5 year intervals, so if 2015 is right for the next test then either:
The born date is in 2010 and the final in 2025
Or
It has already been tested in 2010, and the next test is 2015
Ps at a price of £58 I would guess it has already gone through one 5 year cycle at least, so it either gets retested in 2015 to finalize in 2020 (you have paid a fair price) or 2015 is the final date (and you've only got 2 years life, so a reasonable-ish price based on a life cost of £200)
Note - I've corrected a typo in my first post (2025 final date if made in 2010)
From the view I could not realy say the best size for the cover, some are more stretchy than others, and preferences vary - some people are content to stretch a cover to its limit on a very big bottle, but then its more awkward to view the dates when showing staff at a site.
Someone with the same cylinder may come on and recommend a cover.