Late 60s early 70s fire place

[earthmangreen]

Hello there!

I'm sure the topic has been discussed abundantly for over 20 years, that said I need some advice. The more I read, the more it persuades me away. The task isn't going away and needs doing one way or another!

Late 60's/early 70's build. Previous owner installed an electric fire place and surround. Last year had flies in living room and located them to behind the electric fire. Temporarily covered the opening. Due to the intent of decorating I reluctantly had a look behind. All fine in terms of wild life. After investigating, I presume the previous chap has blocked to suit his electric fire but I can't be sure. I have no foreseeable reason for a wood burning fire but would like to open it up to have the recess. Had a brief look with my torch before I closed it up again. The current opening in fireplace front is only over 400mm but goes around 300mm either way past the surround with what looks like additional blocks to close it up somewhat. What did alarm me was some slithers of board as shown in the pictures. Further reading has lead me to understand that AIBs could have well been used in its construction, not only that but also nasties in the mortars etc. Ive got some TLC to conquer at the roof/chimney level before the fire place gets worked on, however, is this a task you subby out to someone else. Just wanted to board it out and give the illusion of extra space.

Cheers