Fasciation I believe and I've had it in Delphiniums before but other things as well. I copied this for an wxplanation.
"While fasciation in flowers is usually a one-time occurrence, sometimes the fasciation is carried in the plant's genetic material so that it reoccurs from generation to generation"

Yep, fasciation. I've never seen it in a delphinium before, but its one of the plants on the 'more likely to happen' list. Some reading on the subject here

https://www.rhs.org.uk/advice/profile?pid=525

Thanks, hopefully it will grow back fine next year.