Welcome to NGA, CorralAJ. The blooms normally color change and the color progression always ends in a shade of brown so it may be normal. Premature browning of the bloom can also be caused by either very high temperatures, root bound or by inconsistent/insufficient watering. I could only see a piece of the shrub on the picture with a little foliage on the top right hand corner. Those green leaves will eventually turn autumnal colors (say, medium yellows and-or oranges) and then drop. Or if hit by an early frost, the green leaves might turn dark green, almost black, or brown and stay attached longer than those turning yellow. If the shrub now lacks much foliage, it may have had insufficient soil moisture levels before and may develop more leaves as temperatures cool down or once spring arrives. Stems that fail to leaf out as late as by the end of the astronomical spring can then be pruned all the way down. If the shrub's root system is sufficiently developed to plant (could not tell), fall is a good time to plant it. If not sufficiently developed but root bound, you may want to consider potting up. North Wales does not (luckily for you) have a cold enough climate to cause winter hardiness issues for a panicle hydrangea but once planted outside, aim to keep the soil evenly moist at a depth of 10 cm and also keep the soil well mulched (7 to 10 cm) past the drip line; if DR produces large blooms from weak, thin, green stems, consider temporarily staking those green thin stems to minimize drooping by the end of the season (thicker, woody looking stems are normally stronger).
