Does it seem that the compressor is running for long periods and is quite warm/hot to the touch David?
it does seem to run longer that expected and makes unusual noises when it is frosted over
It'll be short on refrigerant David.. Get it topped-up
is this something you can do yourself with the bullet valve and the small gas canasters
If enough R600a remains in the system and you do not need to vac out the appliance, then if you have the correct gauges and you know what pressures you are working to then yes.
You would need the gas; the valve for the gas bottle; the twin hoses; the R600a gauges; bullet valve.
If you do not know the pressures, or can only work by weight, then No, as you would need to vent & vac it out.