Tony Williams wrote:

We had this happen a couple of years ago, exactly as you
described, even to the extent of having to remove the
inner lining and use hair driers. A Hotpoint service
engineer later explained that this was a common problem
with 'frost free' fridge-freezers.

The freezer compartment is kept cold by a fan circulating
air, from the cold coils behind the inner lining and through
the compartment. If the air circulation is blocked or stopped
then two things start to happen, the cold coils ice up, and
the compartment loses its supply of cold air. The icing-up
itself then reduces the air flow, which causes more icing-up,
and so on. Our fridge-freezer had a solid block of ice
behind the inner lining, from top to bottom.

The most common trigger is blocking the air flow by simply
having too much stuff in the freezer compartment. Other
triggers are fan failure or, in your case, leaving the door
slightly open.

Thanks Tony. As you'll have seen from my follow-up, happily all now
OK. Last night I was reconciled to spending most of today getting a
new one, so it's good news. The build-up was nowhere near as bad this
time as last. Then, like you, we had to remove an entire slab of ice.

One thing outstanding: where is the drip pan or whatever it's called?
I just can't remember what happened about the water last time, and
this time the volume has been much smaller. But, as mentioned
previously, I've found nothing I can access to empty anyway.

--
Terry Pinnell
Hobbyist, West Sussex, UK