We have the same problem. The cylinder is empty much faster (after 1 week instead of 4 weeks) and you can always feel CO2 gas coming out when you turn the knob. We don't have a solution yet, but we're not very happy.....
Do the "Sodapop Logan" glass bottles fit the Stelton sparkling water maker? Replacement plastic bottles from Stelton cost between 20 and 40 per bottle!!! and should be replaced every 2 years. The glass bottles cost around 10 and last "forever".
Yes, the appliance is not very efficient. If gas is leaking, the cartridge is not screwed in correctly or there is a fault and the appliance needs to be replaced.