Is dot and dab fixing OK?

No!

The British Standard for tiling, BS5385, the code of practice for fixing wall and floor tiles offers guidance on how tiles should be fixed. The guidance below is taken from The Tile Association guide to DIY fixing, Tile it Right, and encompasses the guidance in the British Standard.

Walls

Spread the wall tile adhesive using a tile adhesive trowel onto the wall. Work in small areas a metre at a time, so that tiles are fixed before the surface of the adhesive forms a skin. Press and twist the wall tiles into the adhesive starting at the bottom and working upwards, one row of whole tiles at a time, using spacers (if required) to ensure a uniform joint. Check the horizontal and vertical lines with the spirit level every few rows.

Floors

Mix floor adhesive as per manufacturers' instructions and spread with the recommended notched trowel to give a ribbed adhesive bed on the surface to be tiled. Butter the back of each tile with a thin layer of adhesive immediately before bedding it firmly into the fresh ribbed adhesive bed with a slight twisting motion to ensure a solid bed and prevent voids under the tile.

Some "solid bedding" trowels and pourable adhesives are available that allow the floor tile to be solidly bedded without the need to butter the back of the tiles. If you want to be sure of a good quality tiling job, use the services of a Tile Association fixer member. Click here to find your local member