tiling large tiles over small

[seamist]

Just a small area (in bathroom) approx 3ft wide floor to ceiling. old 6" tiles, been there about 40 years seem to be fixed firm on to wall.
New tiles are ceramic 30x50, would it be ok to use something like Keraquick or Keraflex? Thanks in advance

 

[tictic]

give the tiles a tap with your knuckles and listen for a hollow sound,if you do hear.. then rip down and start again...

ps
imo just rip down and redo,whats the wall construction?.

 

[seamist]

Thanks tictic

The wall is brick/plaster, tiles seem rock solid, no hollow sound, tried to take one off with a hammer/chisel, did't want to budge.
If I leave on should I try and rough up the surface sandpaper, blade etc?