In view of the age of your home it is unlikely that clips were fitted to retain the insulation slabs against the inner cavity. It is likely that there is a ventilated gap between the slabs and the outer surface of the inner cavity.
Realistically all you can do is to redecorate the wall as and when the block outlines become unacceptable.
Clean a small area of the wall with sugar soap solution then apply a coat of emulsion over a small area to see if the block outline grins through.
If it does you will have to seal the wall with an oil based paint - again try a small area with white undercoat.