The brakes in question are Promac Decode R cable actuated jobs , cable is pulled by tiagra levers. Apparently they have only been used once , they were very dirty so I cleaned the discs , removed the pads and cleaned and sanded them so they have no crud on them.
How did you clean the pads? They're porous, so any use of detergent etc will result in contamination to a depth. This may have happened before you acquired them, of course.
I've known braking to improve after leaving a bike out overnight in the rain* as the running water removes soap residue, so you might gain something by soaking them for a day or two.
I've also had some success removing contamination by soaking them in acetone or meths, and (repeatedly) burning off the residue with a blow torch. There's a risk of heat damaging the glue holding the friction material in place, though, so I couldn't possibly recommend this course of action!
As others have said, new pads is the safe and reliable solution. Lightly sanding the disks can also help if they've become polished.
* This is what led me to realise soap contamination is a problem, and I've avoided washing my MTB ever since!
