Whirlpool side losing cooling efficiency - help!

Hi Hope you can help...

I have a Whirlpool side by side fridge with an Embraco EGY90HLC compressor. The fridge is now 7 years old and in generally good condition. About two months ago the electrics in the house started tripping, this was traced back to the defrost element having an earth leak. I have replaced this successfully (and very carefully in order not to damage the condenser).

A couple of days ago I had an alarm on the front panel that showed an under temp alarm, I reset it and its come back. I now have set up monitoring in the freezer compartment and have identified that the compressor is continually running and it is not hitting its set -18 degrees after its defrost cycle where it goes up to 0 degrees and hitting around -12 degrees after 6 hours of continuous running. BTW the electronic thermostat is within 1 degree of my instrumentation.

The evaporator is clean and not getting particularly hot so I guess it has either lost cooling gas or the compressor is packing up. If so I have the following questions:-

Is it worth re-gassing to last another year or so?
How much would a new compressor be including fitting?

In most circumstances I would by a new fridge however I am due to go back overseas shortly and will only use the fridge for short periods when I return for vacation.