My Neff fridge freezer model K3965 is leaking - Puddles of water are appearing underneath and ice is accumilating at bottom off freezer. I have looked inside and can't see a leak but ocasionly can hear a 'hissing' noise!!! Any ideas?

Your Neff fridge freezer is way overdue for replacement.

draining tube plugged which normally would drain defrost water to a evaporating pan under fridge

Can you remove the freezer rear panel again and take some photos of the frost pattern so I may see what is stopping the cold airflow to the fresh food section?I'm here on Fixya if you have questions
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Freezer /fridge water/ice in fridge and or freezer

Usually due to CONDENSATION due to bad door seals or a plugged evaporator drain line? Can use a soft flexable tubing and very hot water to clean it out. Bad door seals usually need replacement, but u can use a blow dryer and something to pry it closer to the door wall while blowing hot air on to it. To create a seal, to test for bad door seals place a dollar bill in between the door and wall of fridge where it closes. Then slide out the dollar bill with door closed. It should offer some resistance and not be easy to pull out.
Most all fridges have a drain. Look inside your fridge for a "V" shaped channel with a small hole. This hole leads to a tube and a small pan which sits on top of your refrigerator's compressor. The defrost cycle causes water to run into the channel, down the tube, and then it generally evaporates with the heat of the hot condenser/compressor.
However, if that tube is clogged with food crumbs, particles , debris etc, you will have to clear it to get back to a normal mode of operation. A toothpick, pipecleaner, straw or a cotton bud can often do the trick.
Even a frost free freezer will still ice up at times in the areas away from the heated defrost area and the drain line can freeze. This icing causes the pipe work to the compressor to ice over or sweat and cause a puddle. Or if the drainage has frozen over, it will need to be de-iced. ( usually the line is under the freezer bottom plate covering. Towards the back of the unit) And there is another inside the fridge area also.
Give the freezer a good overnight defrost until it's totally clear of ice in addition to making sure the drain is clear to the pan at the back and then restart the freezer.

If it is not from a plugged or misaligned defrost drain tube, then it is always from air leaking into your refer section. Check you door gasket to see if it is sealing correctly all around your door. Make sure your door is shutting tightly every time. Good Luck, Appliance Specialists.

if this is a side by side your defrost drain is froze shut thaw it out with hot water and make sure when you poor water down it runs to the pan underneath the fridge and adjust the freezer temp to 4 degrees above 0

puddles of water appear by the bottom of the fridge every now and then why?

you may want to check your door seal, air leaking thru the door seal causes condesation,also make sure that the drain tube is clear under the defrost element in your freezer.

Sounds like the water hose to the ice maker has come loose. Is it in warranty?

I'm dealing with the same problem right now with the same refrigerator. The cold water tank (for the front dispenser) has a hole in it that leaks every time someone dispenses water from the front door. You will find the water tank behind the top slide-out drawer in the fridge. The leaked water flows down one of the water tubes through a hole in the bottom of the fridge and onto the floor. I'll bet you have the same problem. Good Luck.