Chimney breast getting really hot from my insert stove

Hi everyone

This is my first time posting anything on board, hope i am posting this in the right thread.

I got a insert stove fitted about 3 months ago, It has had very little use. Last week I noticed the wall upstairs was very warm so I checked the chimney breast down stairs and it was extremely hot, So hot that 2 small cracks have appeared on the chimney breast.
The fire had been out for about an hour and a half and the stove was cold yet the chimney breast was untouchable.

I Called the fire brigade as I was scared that there may have been a fire in the chimney breast (thankfully there wasn't)
Even the fire men seemed puzzled on how it was so hot and no fire.

The plumber who fitted it has never seen this happen before. He has moved back the flu and insulated it even more but unfortunately it is still happening.

The stove is an Antrim 5kw
An hour after a fire has gone out the stove will be cold I thought stoves were meant to retain the heat.

Any advice is much appreciated