Devonstew yours were sown and planted at good times – maybe too close to the large psb? I gave mine more space this year, one row in a 40 inch bed, 24 inches apart. Variety Doric F1, excellent large sprouts all winter, so they have liked the conditions, especially the mildness, but also do need staking as Bellamax says.
Like all brassicas they like heavy, dense soil and Devonstew are you on lighter sandstone? May need extra organic matter….