Sounds like the drain tube has come adrift from the gutter that collects the defrosted water and feeds it to the drain hose. The water has frozen because it hasnt drained away and when the plate melts on the next defrost cycle it is filling up the top compartment making things even worse. Defrost the unit completely over 12-24 hours with the doors open and see if you can line up the drain tube back to the pipe that feeds it and reconnect it. Until you do that, you will keep experiencing the same symptoms/problems because the water wont escape and it will just freeze up and do the same thing all over again.
You can also try running a little hot water down through the drain tube to clean it and confirm it is connected correctly, If you cant seem to fix it up, it maight pay to get someone out who has a lot of experience with fridges. Its worth it just for something like that. Ring around, explain whats going on and Im sure someone will do the right thing and help you out.
Thanks for that advice. I now have to work out how to remove the rear panel so that I can get at the tube but I won't be able to tackle that task for a couple of days. At least that will allow me to plan for the 12-24 hour shutdown (I have a stand alone Deep Freezer so that won't be too much of an inconvenience).
Take out any shelves inside the freezer where the main cooling element is. You might have to remove the grub brackets attached to the inside of the freezer compartment that the shelves sit on, if your shelves dont just slide into the unit(Some vary). The next step is to remove all the screws across the back panel, you might have to remove little white plastic caps that cover them as they are hidden for visual appearance. Remove any air vent cover screws that the fan blows through if one is visible. Sometimes there is a screw or 2 attaching the plate to the rear wall of the freezer there as well. Once you have removed all the screws that hold the inside panel on the rear, the main cover should lift forward and you will have to unplu any wires that are connected to it, such as the earth and possibly the fan wires, depending on whether the fan is attached to the plate itself, or the rear wall( it varies). Make a not of where the wires go and dont forget so you can plug them all back in afterwards when you re assemble all this. Once you have the cover removed, you will see down the bottom, below the cooling element is a gutter and in the middle is a drain hole for the water to escape. See if there ice build up there, and or any obstruction blocking its operation of water removal. Once clear, run whipper snipper cord down the drain and twist it around to clear it as well.
Before you do any of this, inside the fridge compartment, i want you to trace the origin of where the water is leaking through from up above. If its in the middle, right up the top and back of the fridge, you'll need to remove any temperature control box up there by undoing the screws and un clipping the unit so you can gain access to the rear of it. Sometimes, the drain tube goes down to a drip tray, which collects the water inside the fridge compartment up there, and then feeds water into the drain hose down to the motor. The little drip tray often get clogged up with gunk and before you do the whole defrost thing, its best to check this first, as it may be all that is wrong in your case. The tray can simply be pulled out of the hole in the back wall, once you have access to it, cleaned and placed back into position. While you have it out, run your wipper snipper cord or something similar down through the hole and it should go all the way through to the motor. I was thinking about this earlier, and I reckon this is where your blockage is more likely to be because the ice is still melting in the freezer compartment and if its making its way down to the fridge compartment, the top musnt really be restricted. Check for the drip tray inside the fridge, hidden up the top, middle and furthermost point to the rear inside the fridge, they usually are hidden by the temperature control box and you'll need to stick your head in there, look upwards to see where the screws are to be removed. Its worth a shot and might just save you a lot of hassle down the track. Let me know how you go okay.
Thanks for that wealth of information.
I won't have a chance to tackle the job until the weekend and I will let you know how I get on. Hopefully I won't have to take the back off the refrigerator because it doesn't have any screws visible - it looks like it is glued on.