Thanks for the feedback on sleep systems.
Being impulsive, I've almost ordered an OR Helium bivy (reviews are extremely positive). But as I must also try to be reasonable, I'll probably try a dry run and see how things fit in my panniers before pulling the trigger. And there is this thing about rain (a certainty for this summer) which might make me regret this decision. (OTOH -- what a nice piece of equipment
)