Samsung Fridge Freezer RL39WBMS - Fridge Not Cooling But Freezer Working Fine
Sorted. 

One of the interesting 'features' of this frost free freezer is that it builds up ice at the bottom so that frost doesn't form throughout. Although this means that you need to get the hammer and chisel out on a regular basis to remove the iceberg from the bottom of the freezer roughly every few months. 

On this occasion I was a little slow to get around to doing this. Believe it or not, de-icing a freezer is rarely on the top of my priority list. Anyhow. Thinking back to when the fridge broke and before I replaced the temperature sensor, I remembered that I de-iced an enormous iceberg which had actually grown up behind the bottom drawer to about 15cm tall which I chiselled away as much of the ice that I could get to. 

And then it triggered, a note on this fridge at the back mentioned that it was a "multi-flow". Didn't really know what that meant but thought it must mean that there is one cooling system which flows cold air between the freezer and the fridge. Since the back of the unit it basically a completely solid unit, it didn't look like I could do much there. 

I noticed a few screws on the back of the freezer panel on the inside of the freezer though. So I turned off the electricity (safety first...) and decided to unscrew everything. Naturally when you have all the food out of the freezer, time is of the essence. As such, a hammer and chisel ( or screwdriver) come in really handy here and worst case scenario you puncture a few holes in the unit (whoops...). A heat gun is also a cracking tool that you really need to do this as well, thankfully a random purchase on ebay a few years ago came in handy :-) . 

Back to the point though, basically there are two plastic panels at the back which come of. Behind these two plastic panels, when you finally get them off by chiselling the ice and melting it, you'll see the cooling unit at the bottom. For me, this was completely iced up at the bottom inch of the unit. After much de-icing, I noticed that behind the metal panel about 2 inch wide, towards the bottom right of the unit, this is actually covering a small vent. This vent was also blocked full of ice. So I de-iced that until I could finally stick my fingers up inside it to confirm there was no more ice there. 

Then just screw everything back together and bob's your uncle. Job done. The fridge is now cooling again. 

Just a note to Samsung, this is a really poor design. This appliance needs too much TLC, it should just work without needing regular attendance. And it is an absolute nightmare to sort when things like this happen (due to your shoddy design). But hey. 

If you don't have the skills/confidence to give this a go yourself, you should be able to basically fix this problem by turning the appliance off for about 2 weeks (keeping the doors open) and the melting process will happen naturally in a normal household. Your food on the other hand may not quite be in a usable state by the end of those 2 weeks sat at room temperature.... 

I think I'll be adding Fridge/Freezer Repair Man to my CV!

@Ed Beal - By the sounds of things that would likely also exhibit the same problem, thanks for the suggestion. For those wondering and if in doubt that the fan may have gone, once you take off the first panel I mentioned above, then you can see the fan, so if you turn the device back on you can check that the fan is spinning (make you've nothing touching the electrics though... electricity, water and you mixing rarely ends well...)