How to fix cracked wall/plaster under lintel?

[milanr_97]

Deep crack on the inside of an exterior wall, running along (pretty much the entire length of the) underside of lintel, above a recessed window.

Seems the crack is between the lintel and whatever's been used to finish the wall (plasterboard?) on the lintel's underside - no cracking on outside. There's a mould issue around this area so initially thought the crack was from moisture etc. but now think that whatever adhesive was holding the board to the underside of the lintel has given way in places and gravity's been worsening the crack so that it now extends as far as the lintel's length and depth - was able to insert a 6" knife all the way into the crack.

Board is sagging and need to get it fixed before it completely falls off! Want to know if I need to get a professional in to completely replace the board or if I can seal the crack with anything and just finish with polyfilla, sand and paint over?

Thanks in advance for any and all help!

 

[noseall]

Weve got some ratchet style board props that can be lengthened or shortened and would be ideal along with a flat straight edge board beneath that header reveal, in order to squeeze it back into position. One could flatten a foam nozzle, squirt small amounts along the length of the crack, about 50mm in, then press the flat board into place with the ratchet props. Sheet everything though because of the excess.

If it is sand cement render or similar it may just crack though.