You can plant it now. Now is actually the best time since the growing season is winding down and lots of bargains at the garden center as they start pushing to liquidate their stock. Since you haven't bought one yet, take a look at 'LA Dreamin". If you can't decide which color to buy, LA Dreamin might be the answer.

it is ideal at the moment as the ground is still warm and there is time for it to settle in before the frosts. make sure the hole is at least x2 the size of the pot it comes out of and back fill with the soil and added compost if you can to help the roots explore . if the plant is root bound gently loosen the roots so they don't continue to grow in the way they are in the pot.

I note you're in the Home counties - that means you can plant right up to end of October, and in fact, the nearer London you are, it would be best planted between late September and the first week of October, because its been so dry in the south. Work in some composted manure or good garden compost to the area you want to plant it in beforehand, preferably a week before you want to plant. When you do plant, water in well...

As the name implies, hydrangeas love their water. Be sure to water in well and keep the soil moist if you don't get enough rain. A couple inches of organic mulch on top can't hurt.

Thanks everyone for your answers, really helpful,my ground is really dry at the moment so I think I will leave for a few weeks and make sure I prepare the soil.