I would say now, as I bought some already growing Hollyhock from the GC. They are still small, so this must be the right time of year

some are biennial, sow the seed now and plant them when they have at least 4 true leaves where you'd like them to flower. They may reward you this year if not then next year.

I have them germinating all over the garden as we speak, so now would be good, maybe under a cloche.

it should tell you on the back of the seed packet

Grow them on until they are nice and large or they can 'disappear' after planting.

Many thanks for your helpful suggestions.