Forgot to say that there is no frost build up. There is a gurgling sound at the bottom of the freezer - sounds like water sloshing around but I have no idea if this is normal.

Having unscrewed the front screw to the ice making tray it seems the back screws are not screws but just posts so it came out easily and gave me access to the fan housing.

Once I removed that the fan started up and seems to be working fine - the voltage tests are fine but as the fan is now running they would be. However, it wasn't running before so perhaps in the time it took me to unscrew everything something defrosted/warmed up and allowed the fan to run properly.

Is this a thermistor problem or could it be something else?

I'll leave it running (without ice maker reinstalled) to see if everything cools properly. There was no noticeable frost on the evaporator fins although some water did drip during the warm up time - but only a couple of spoonfuls.

Your model number seems to be missing several digits at the end but here is the parts listing for this series.
Replacement parts for GSS25XGP models | AppliancePartsPros.com

Here is am manual which may help.
Since it is a GE and they like to keep all tech info a secret, this is the best I can do.
31-9072 Arctica Profile GE Side-by-Side Refrigerator Service Manual - ApplianceDigest.com
You have to join the site to download the manual but it is free and this is a very good site for manuals and other info for the appliance repair DIYer.

The evaporator has some frost but just a fine dusting - is this a problem?
No a light coating is normal after the unit has run for a couple hours.
As a matter of fact a light even coating is a good sign as it tells you that the sealed system is OK.

For defrost to be a problem the evaporator coils have to be frosted/iced up to the point where the evaporator fan cannot pull air through them so the unit does not cool properly.
Also it normally will not clear itself re: you have to do a manual defrost to get the unit running OK again.
Of coarse if the defrost has gone intermittent then it could clear itself.
A possible cause of intermittent defrost is the defrost thermostat (Item 240 in Section 3). Sometimes you can see that it is a problem as it will be deformed/bulged due to water getting into it.

You problem sounds more like a evaporator fan problem.
If the fan does not work then there is no air circulation and the cold air collects at the bottom of the fresh food and freezer sections.

GE is well know for problems in this area just take a look at the following post/thread.
http://forum.appliancepartspros...or-test-replacement.html