You need an approved Idest centre:
http://www.sita.org.uk/idest/idest_members.php
For somewhere local there is likely to be a centre somewhere nearby, but failing that dive shops will generaly take a cylinder and arrange testing. If they are paintball friendly this then saves you the postage (and the difficulty of finding a method suitable for postage of cylinders)
If you can get to an appropriate venue then handing of over to hpac is preferable as they are the paintball air experts
Typically a test lasts 5 years, there are some variations depending on cylinder design specifics
An aluminium cylinder is valid in the uk for 10 years, though markings will say 5
A steel cylinder is valid for 5 years, but is a rare sight. What people typically call a steelie is usually aluminum
Steel and aluminium cylinders will last forever if they are retested as required. But at about £30 new aren't necessarily worth the effort
Batter a steel/aluminium cylinder and you just scratch the paint finish
Fibres have a maximum 15 year lifetime and are marked with a death/expiry date. They require tests every 5 years
Stakos are an ultra light design and cannot be tested, so have a 5 year lifetime
There are new ultra light designs such as safer & fuel, these can be tested but have a maximum 10 year lifetime
Batter a fibre cylinder and you damage the fibre. If minor on the top finish then it's fine, if it gets to the fibre then the cylinder has become a paperweight
Check this out:
http://www.oaklandsfestival.host56.com/web_documents/air information.pdf
Also check out this episode with cylinder info
http://thepaintballshow.tv/thursday-31st-oct/4580355983
