in my experience if they are fully hardy species I repot into 0.5l pots and bring them on and plant out late autumn when ground still warm. though sometimes I just pot them on and wait until spring.

just a word of caution though 9cm pots are not that big and the plants may have been in polytunnels so not hardened off for a typical late Sept /early Oct frost.

Beware imports too. I've unwittingly bought a couple of things on line lately that had import certificates on them.Using imported stock was the cause of all the ash dieback so it pays to be careful.

A lot of these 9cm potted plants are 'Liners' which nurseries and some centres buy in to pot-on. I would check to see if they are rooted and take Eileen's advice and pot them on into a larger pot, perhaps 12 to 15 cm and overwinter them in a greenhouse or cold frame.

Well, Thank you all three! I don't have a greenhouse or cold frame. I hadn't thought about checking that they are rooted, or about the fact that they might have been grown in poly tunnels and I certainly wouldn't want to buy imports. Thanks for the heads up on all these points! I will leave it for now.