After looking through some old negatives, I decided to give a set of particularly bad ones (marked 'burn') another chance. This was a roll that had been sent off to my lab for development and received N-1 in XTOL.
At the time they were put in my throw away batch for aesthetic reasons, but I hadn't noticed this until now:
They have been stored exactly the same as all my other negatives - about 30 rolls in this box - and are the only ones with this problem. Several of the other rolls in the box were developed by the lab exactly the same, and shot on the same film - Delta 100 rated at 50. However, the roll in question (from which the attached image is sampled) was sent off with my first roll of Delta 400, rated at 200. Is there a chance they were developed together by the lab? Or is this something to with storage - possible condensation/heat? As I say, I didn't notice it at the time and certainly would have.