Replacement window boards in conservatory

[AdamD]

Our conservatory is 20-25 years old and in need of a bit of a spruce up so I am replacing the 3 window boards. However, underneath is not a particularly good work area to get them level so wondering if you can offer advice? I would put wooden wedges in to keep everything level but there is no ledge from the brickwork under the window to rest them on and screw them to. Also, without screwing what is the best way to secure the boards? No more nails adhesive?

 

Pics below. Please ask any more questions or request more pics if it helps shape an answer.

 

Also, slight cracking of the tender when I removed the old ones so what would you suggest filling that with?

[AdamD]

This is the cracking. Bear in mind the outer edge of the board will be fixed down so the cracking will not be as much as shown here (other than the biggest bits). The board currently slopes down towards the window so I need to bring it back level.
 

Edited May 5, 2022 by AdamD

[Simplysimon]

window sills used to slope towards the window due to condensation on the glass running down. if it sloped the other way then the water would run onto the furnishings. a deeper sill protruding further into the room will allow an apron to be placed under the sill to hide any gaps or missing  render. a thin bed of mortar would bed the new sill if it needs that much or a tube adhesive if less. the adhesive may be sufficient to hold it but would be better with some screws or dook the joints and nail it down.

[tonyshouse]

I like to sit the window board on the ledge formed by the top of the back of the sill

 

like to fill the cavity with platinum EPS beads

 

like biscuit joints in the mitres, love gorilla glue and packing to perfect position weighting it all in place and foaming in all the gaps