On the back of the freezer compartment, you will see a removable panel. Prise the small round cover cover off the panel & unscrew the screw. On removal you will see some coils. Look at the bottom of the coils & if any water or ice is present, your drain tube is blocked!
Take off the lower panel at rear of appliance & you will see a plastic drain tube (mine was corrugated pipe), twist off at the top & pull off at the bottom. Pipe will almost certainly be blocked, give a good clean & check also the top hole where the pipe fits prod something up to clear (I used a chop-stick). I also used a hairdrier to melt the ice in the coil compartment before replacing the cover.
My drain tube was totally blocked up. Hope this helps someone.