Usual answer:
Send it to HPAC (most expensive method due to post there & back)
Hand it to HPAC at an event
Ask the site you go to if you can hand it to them to arrange testing
Find a local dive shop and hand it to them to arrange testing
think it cost £24, i believe the bottles go for testing on thursday, come back tues or wens, not certain on that, as it was a few months back i went there, hope that helps tho!
regards
psycho