What bod said
Small aluminiums like a 13ci are exempt
In America they give 2x2 as a guide (within 2 inch diameter x 2 foot tall). That's not the same as the U.K., but gives you an idea
If you can find someone local to test it then it could be worth the money to test an aluminium cylinder
(Note that the markings on an aluminium may still say 5 years, but they are U.K. legal for 10 years, and if you do test them they have no final date)
You could get lucky with someone who will test for £15 - £20 as long as there is no postage involved
Check the IDEST members list
https://www.sita.org.uk/idest/idest_members.php
However I was charged a nice price for testing my fibre cylinder at a local dive centre, I went back to ask about prices if I took them 5 aluminiums
But this time the other guy didn't like paintballers and offered a ridiculous price
I never bothered to go back