Looks like mealy bugs to me. Try insecticidal soap or use Neem oil as a repellant on the unaffected plants.

They'll be lupin aphid - you can rub them off with your fingers, use neem spray, or any insecticide spray. If you use the latter, bear in mind a contact insecticide will need repeating frequently, so monitor the plants regularly - don't spray open flowers, and preferably,spray at dusk when bees have stopped working. If you use a systemic spray, only use as often as it says on the container, again avoiding open flowers and using when bees aren't active.

Oops - Bamboo got there first.

Are they actually a greyish-white colour, with the sun just making them look whiter in the picture?

If so, they could be Lupin aphid. See RHS page for info:
https://www.rhs.org.uk/advice/profile?PID=653

Many thanks to all responders for your advice. I did try just rubbing them off at first but they multiplied too rapidly. I have since been using an insecticide although I don't like doing that as there are many bees around. I spray late at night but next day the lupins are still overrun. Hard to get to all areas as there is dense undergrowth. Should I perhaps just cut them back to 12" and spray down to ground level and hope they will grow back again? The bugs do not appear to be attacking adjacent plants but have found another lupin some distance away, so I guess they are the lupin aphids.

I used Roseclear ultra, a systemic (as opposed to contact) insecticide last year - had to spray them about every 4 weeks, so it worked quite well.

I have just read your post I had mealy bug on my streptocarpus and I used meths painted on with brush
It was a great sucess