Fixing slate fire surround

[Cloudscape]

My house has four simple slate fire surrounds. Each consists of four pieces - two uprights, a 'lintel', and a shelf. One on fireplace, the shelf is firm, but angled down (so anything placed on it would slide off). The other three pieces are completely loose - not attached to the wall at all.

How do I level up the shelf? Should I expect to find that the shelf extends a fair way into the wall? Do I use plaster or mortar to refix it?

How do I attach the rest of the surround to the wall (brick, I think)? It appears to have been previously stuck back with plaster, and this has failed.

 

[Lime]

For most slate fireplaces that I have moved/repaired the shelf is just sitting on the lintel and uprights using the plaster on the wall to hold/wedge it in place.
(It should extend to and touch the bricks under the plaster.)
It probably has no other fixing.

Often the rest of the slate parts have been just stuck on the wall with plaster of paris and are usually glued together using it too.

If the shelf just needs altering, rake out the plaster directly on top of the shelf to the depth of the bricks and about as wide as a medium screwdriver, holding the shelf all the while.
Avoid scratching the slate.
(sticky tape will protect it.)
Carefully move the shelf up and down to check it is free of any other sticking bits and pull away from the wall.
As I was once caught out in a similar situation, make sure someone else holds the lintel and uprights against the wall when you remove it!
Clean the slate and hole from which it came and replace the shelf as you want it using plaster of paris to fix it with.

If the other parts need refixing clean them carefully as well as the wall and refix with plaster of paris.
Plaster of paris sets quickly and works well for this application.
don't be tempted to use cement.

 

[Cloudscape]

The shelf is rigidly fixed (at the interesting angle). It would stay put if you remove the three main fire-surround pieces from beneath it, so I am guessing it must extend into the wall. I did wonder what would set quickly enough to do the job of fixing it all back. Thanks for the advice.