Hi Jonny
All my diving kit is in regular use and is cleaned and tested anyway, so its only the cost of the DIN to Bullnose adapter (£22 from SAP) and the oxygern reg/flashback.
207 bar alu 40 and 80cf cylinders are still in common use as deco bottles, I typically carry two 'Ali80's' (11.2 litre aluminum cylinders, 207 bar WP) for a 70m dive, one with 50% oxygen and the other with 100% for the 6m stop.
In the garage I have 2x12 litre steel 232bar single cylinders, a 2x12l 232bar steel twinset, 2x15l 232bar steel twinset (a real back killer), 3l 232bar ali, 5.7l 207bar ali, 7l 232bar ali and two 11.2l 207bar ali cylinders.
All are in test and O2 clean, so I was thinking I could use pretty much any of them for O2 at 1p per litre from the dive centre.