Thanks to Leroy and X4 for the further info that has been added to this file.
It states in the manual that the air con must be used for at least 15 minutes
every week otherwise the rubber seals will perish and the gasses will escape,
no gas, no air con. Use it or lose it.
OK, I Lost It, What Now?
You have 2 options, DIY or get a professional to do it.
The info you will need to arm yourself is as follows.
Roadster = 648g +180cc compressor oil
Forfour = 430 + 100cc compressor oil
Get a Professional
Hook out the yellow pages and get ringing, it doesn't cost a lot to do and the smart is a relatively easy system to refill. This way you know the system has been drained and everything has been done properly.
Do It Yourself
Thanks to Martin Lowe for the two pictures below.
Remove the Roadster boot liner by following these instructions.
This is the high pressure valve, you should not use this to fill up using a DIY can of gas.
Again, the valve you want is a little more hidden. Look just behind the battery.
Connect up the air conditioning refilling can and fill until the dial shows green.
Always read the instructions on the can in detail and ensure you follow each step.
Remember that DIY isn't ideal, the gas has escaped but the oil may still be there,
adding more gas/oil may cause problems with there being too much oil.
Just adding more gas may not work long term. The new gas may escape from
the same place as before. You must cure the fault before the gas will stay put.