Any advice regarding tiling on top of tiles in my bathroom.
How many kilo grams of adhesive would I need per square metre?
How many kilo grams of grout would I also need per square metre?
The tile measure 20 cm x 20 cm.
Any advice appreciated.
Kind regards.

For tiling over existing tiles you'll need an adhesive specifically manufactured to do just that or you'll need to remove 80% of the glaze which would take as long as removing all the tiles. ABA/Ardex make "Ardion 90" or "Optima" both are suitable for the purpose.
With a 6mm notched trowel you'll use 2kg per m2 adhesive and .25k per m2 grout that's a 200x200mm tile with a 1.5 mm grout joint
Ask at the tile shop if the adhesive they sell is suitable for tiling over existing tiles.

A good source of advice for DIY questions is The Motley Fool DIY board:
http://boards.fool.co.uk/messages.as...0322&bid=50975

There's loads of other advice covering most issues on other boards, too.