it looks like weed killer spray that has drifted on to it.
failing that the catch all is probably a virus.

It looks like lack of water to me. Leaves droop and develop chlorosis. I would water it well and add a feed and see what happens after a week.

That's viral. I suspect cucumber mosaic virus, or a related pathogen. Dig it up, burn it, don't handle other plants after handling that one. Avoid planting delphiniums in the same spot. (That can be awkward when it's the best place for them!). Failing that, new compost (good stuff), and keep new plants strong and healthy.

there appears to be a healthy white one behind it. so keep your eye on that one too.

Going to disagree - though the OP can advise if the soil is dry or not. Delphiniums go droopy and develop chlorosis from the lower leaves up due to lack of water.

thanks for trying to help everyone ! I don't think it's a lack of water or drifted weedkiller although I have given consideration to both ideas. The surrounding plants are doing well and I think either option would have affected more than this individual plant. Which only leaves Worthy1s option of a virus. I really can't bear to dig it up and burn it (yet) as I have nurtured it from a seedling so I have cut off all affected leaves and will give give it another couple of weeks to see how it goes. My magnificent delphinium now looks like a stick of celery - I could cry I so don't want to lose it