I had the same issue, a pool of ice at the bottom of the freezer compartment. I fixed the problem in a similar way to Peabrain's solution posted here.
I download the service manual and whilst the drain pipe at the back was dry and unblocked, I took the internal shelves out and removed panel 332A (hard to pull off because of ice and clips- be careful of the clips!), then panel 330A, and discovered ice at the bottom blocking the draining hole at the lower left (hidden until the ice was removed).
Once exposed I put hot water down that hole, the blockage was cleared and the water flowed out the drain pipe at the rear. Whilst I broke a couple of clips removing the panels, I got them all back in together and the problem is now fixed.
Thank you Peabrain!
dextrovix, August 2012