this e has a comprehensive set of instructions.

https://www.gardeningknowhow.com › Ornamental Gardens › Flowers › Hibiscus

Thank you :)

Sbg, those are instructions for Tropical Hibiscus (Hibiscus rosa-sinensis), rather than the Hibiscus syriacus that Hywel has. In the U.S., we call Hywel's the Rose of Sharon, though I understand that that name is used for a kind of Hypericum, there in the UK.
As for pruning, I would prune it back hard in the early spring, before it leafs out, and it should grow back quickly. Blooming may be delayed by a couple of months, but you can speed that up by applying washed seaweed at pruning.

I've had a couple of these for over 20 years. As Tugbrethil says, early spring is best, but you can really prune them anytime you want. It won't hurt them. I have them trained as a hedge and give it a trim about one per month. They are quite vigorous.

the article also said spring was best. !

Thank you everyone. All information is helpful :)