Regs state that you cannot pass a 4" sphere through the widest point. Designed to stop accidents with small children etc...
This is part of the spec i have for my loft at home...
"Balustrades to landings (and at side of stairway if applicable) to be min 900mm high, and to be capable of resisting loadings. No openings permitted which permits passage of 100mm dia sphere. Bottom edge of balustrade to be n.m.t 50mm above pitch line."