Ideas for north facing patio.

You could try small, low rockery plants. They tend to be slow growing to begin with but do look pretty and if you keep them under control you, can prevent them from encroaching too much onto the flag stones.

Ferns come in lots of shapes and colours and are ideally suited to low light conditions. If it is colour you are looking for Bizzie Lizzie is perfect.

A north facing patio is to be envied in a hot summer. I would fill in the gaps in the paving go for pots of topiary and ferns.

Think quiet, still, shady calm!

Prince Charles has a paved thyme walk. I checked on Google that it will grow in the shade, and it would smell lovely when walked on.
I agree ferns would be suitable.

I agree about Bizzie Lizzies for colour. I like growing them in an old galvanised bucket but this year I couldn't get the plants.

Hydrangeas do well in a North facing situation. This year my Osteospermum overwintered in a planter and are now huge. Bedding begonias also seem to love the aspect.

Meant to add that one year I planted Mimulus, the seeds scattered and now little plants pop up between the paving. They’re lovely. Of course plants growing in cracks are not to everyone’s taste.