how close should the space be between porclain?

[Pawelzik]

...bs5385 recommends a 3mm joint but i think 2mm can be used .....

Yes Dave, same here in Germany acc. to our national rules DIN 18157, 2 mm minimum recommended, 3 mm and more is normal, Martin:8:

 

[sWe]

Swedish standards state 3mm for wall tiles, and 5mm for floor tiles. They also state that the width can be increased or decreased once the type of tiles used and the substrate are taken into account. Essentially, anything goes, as long as it's appropriate and structurally sound.

I always advice against butting together, but I've done a few such jobs. I'd only ever do it indoors, with good quality caliberated tiles, on a solid (old) concrete substrate though. I'd never EVER do it on board materials of any kind, or on concrete younger than 3 years.

 

[Droopy]

I just thought I'd add my tuppence worth to this.

I would NEVER butt tiles together, no matter how insistant the customer was.

3mm min for large floors, 2mm min for small bathroom type floors. Even then, that would only be with top quality rectified tiles.