I find the advice available about the spacing of lily bulbs quite confusing. The RHS suggest 30cm apart and Gardener's World and Sarah Raven suggest 15cm. I've even seen suggestions of 8cm to 12cm for commercial cut flower production. I've spaced them at 30cm in the past and they've been fine but I wonder could I shorten the spacing to 20cm or even 15cm and get a lot more in the same spot. Has anyone got experience of growing lilies for cut flowers?