Septic Tank Emptying

I just wondered what diameter pipe is needed for septic tank emptying tankers? Is 110mm adequate or should I go to a larger size?
For the tank I'm using I plan to install 3 emptying points, the centre one to be vented, I thought about using this for the other 2:

http://www.screwfix.com/p/floplast-screwed-access-cap/72385

I'm not too worried about regulations, as whatever I do will be better than what is there now. There is a bathroom with a waste pipe that is just emptying straight onto the land! In UK this would be deemed unfit for human habitation but my house is in a very rural location in Croatia.