Fixing a curtain pole to a bad wall

[t1tch2]

Hi all

Ive just brought my parents house and always remembered their curtain pole hanging from the wall as i grew up. Now im older i thought id try do a better job. Now i know why it always drooped, the walls are thermalite blocks covered with plaster and above the patio doors its almost as if it is hollow its so bad. ive tried putting up battoning but again with no luck.

Any help/suggestion greatly appreciated.

Steve