In general , you don't mix refrigerants as they have different characteristics, even if it is classed as a "drop in " replacement. Even if it were a drop in replacement the system would need to be purged and cleaned prior to it being re-gassed. In addition "drop in" replacement refrigerants do...
Have thought about buying an inset gas fire, but am unsure whether i need a flue liner or not. The existing chimney has a beige ceramic type pot of about 6 -8inches which appears to go to the roof. No other fires are connected to it.