Samsung Fridge/Freezer (Frost free) - Ice forms at bottom

We have a Samsung fridge/freezer model RL36SBMS. This is a frost free model. Over the last couple of months ice has been forming at the bottom of the freezer section. It looks like water is leaking from somewhere and freezing on the bottom. There is no water supply to the appliance and I have left the ice tray empty so it cant be coming from there. I remove a section of ice about 12" x 6" x 1/2 " inch thick about every two to three weeks. Any ideas please.

There may be a drain that's supposed to allow the condensate to flow away from the evaporator during normal self-defrosting operation. if it's blocked the accumulation would overflow, then freeze.

Thanks for the tip. It has taken me all this time to get round to looking at the fridge/freezer. Having worked out how to get to the workings from inside the freezer section I found that any spare space was totaly blocked by ice. I have defrosted it all and after some effort the small drainage tube has now unblocked.
Thanks for the advice. It has saved waiting in for a day for an engineer.