You dont want the cone shape on the CO2 adapter, it needs to be a flat surface (normally with a sealing washer).

The M25 valves are very difficult to get hold of now days for some reason. You can get the 3/4BSP ones fairly easily though, so might be worth changing your bottle if possible.

Is your regulator mounting directly on to the bottle or do you have a hose inbetween (as if so you will also need a 1000psi relief valve on the high pressure side)?

As a back up plan, you can modify the fire exinguisher handle, drill a hole through both parts of the handle and put a bolt through with a wing nut, as you tighten the wing nut it squeezes the handles together.