I have a dive tank I haven't used in an age. Mint condition, with surface/land use only (obviously), and always kept inside the house never in a damp garage. I know it's out of date, and therefore would need a re-test. I think it's a 15 liter (I can check if you like) and rated for 3000psi use only.
More than willing to sell for less than the £100.00 you have stated, as it needs a re-test. I'll include the clamp fitting as well.
LMK, via PM if you're interested.