Dave wrote:
Been away for a few days and just noticed on our return that the fridge has
started acting almost like a freezer. The back panel has maybe an 1/8th to a
1/4 inch of frost on it and things such as two bottles of lemonade that were
very lightly in contact with the panel have frozen solid.

It's a Hitachi Larder Fridge (Model R11 LCS) and, when we bought it about 10
years ago, we set the temperature dial to "6" and it's never been moved from
that setting at all, ever.

Any idea what's wrong and if it's diy-fixable?

Dave.

Turn it to "3"?

If the cooling elements run in the back then you can get localised
freezing.