Looks like fungal spot. depending on which fungus the lesions are either dark brown or black. I fungicidal spray should help.
discard any infected leaves in the household bin and don't plant any new lupin plants in the same place for a couple of years.

Thanks, Seaburngirl. Does the plant need to be lifted and destroyed too?

if you don't treat it with a fungcide then personally I would.

I'll treat it with the fungicide. I have brought these on from seed and it has been a real problem given the conditions this year. Most failed and I was left with only a few plants.

I checked the other plants which are spread over the rest of the garden and several others also had these spots. So, I suspect it has not been the soil but the conditions outside. Where we are, it has been extremely dry and not very warm.

So, I have removed the leaves and will spray the plants tomorrow. The rain came on after spraying the first one this afternoon. So, did not get to finish the others.