Hello, If the refrigerant pipe is not fully frosted up to the bottom pipe, this is a symptom of a refrigerant leak. But in some cases, if there is a damaged door gasket, fresh air could seep inside the freezer and frost would accumulate faster than it should.
I would suggest, defrost the whole freezer first, check the door seal all around, specially the bottom part which is hard to see, then plug it back and observe. If , after 24 hours and the door seal is fine, and still the refrigerant pipe is not fully frosted up to the bottom pipe, then I would say, you have a refrigerant gas problem.
If this is your case, I am sorry but this is not a DIY. You would need a professional to do the repair and you should get quotes first because this could be an expensive repair. It may be worth just replacing the unit.
Since my original question, I have managed to borrow a meter from a friend. I have removed the defrost heater and sensor from the freezer. There is a resistance showing on the heater (as you would expect). There is infinite resistance shown on the sensor when it is warm, i.e. open circuit and there is a low resistance, so it would appear to be working.
The heater and sensor have been removed for approx 2 hours now and the freezer is down to approx 0 degrees. Frost has formed on the pipe leading to the top of the coil, and is starting to form on the top of the coal and fins in the top corner of the coiul where the refrigerant pipe enters.
As you suggest, I will leave overnight (with the heater still removed) and see if more frost has formed. If it hasn't - would you suspect a refrigerant leak?
Also - I have checked the door seals and these all appear to be OK.