It dont think it matters whether your vehicle is being used for commercial puposes or not (although it may be an insurance requirement).

The requirement is based on the ammount of air/cylinders you are carrying.

I looked into it some time back with the HSE, it was something daft like 300litres ONO.

You would have to really be packing some to make it a requirement.

My Step Daughter and son in law are coppers, and I have mates who are firemen, and I would like to think if there was a crunch, the emergency services would at least have a clue what they were dealing with.

It is not a requirement, but aside of the theft issue, I think it is a good move to display a sticker, magnetic idealy so you can bung it on the cylinder, and transfer it to the motor when it is in residence as said.

I just used to keep forgetting the bloody thing!!