What happens if I don't return all or any of the used gas cylinders after placing an order for full gas cylinders under the plan?

The gas cylinders are the property of SodaStream and you'll need to return them after use.
If we don't receive 4 used gas cylinders within 30 days of ordering full ones, you will be charged £10 per each unreturned gas cylinder. If you fail to return the gas cylinders during such 30 days and return them later, it would still be possible to get a refund of the £10 charge, but it's best to avoid the inconvenience.