I would leave the dead flowers untl next April and then cut them off. It's not a disaster as the Hydrangea you have will flower on next years non flowering growth.
However, you must water it as much as you can now as it will go brittle and die if it dehydrates. Winter should take care of it, but if dry weather comes water it and it will thank you for it and flower well next year.
Outs is exactly the same, usually has a mass of lovelly big pink flowers, I had been watering it, didnt seem to help, but we have had some decent showers on and off now the past week.
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"We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
However, you must water it as much as you can now as it will go brittle and die if it dehydrates. Winter should take care of it, but if dry weather comes water it and it will thank you for it and flower well next year.