Our nan has just moved into a new disabled flat and the whole place is dry
lined (hollow walls).

She has two 3.6m wide windows which need curtains and poles / tracks.

There's usually a lintel above the window opening. Have you investigated
this? You may find you actually need something more suited to reinforced
concrete. (i.e. SDS drill, decent 6 or 7mm SDS twist bit and some rawl
plugs).

Christian.