Could it have been sprayed with weedkiller by mistake?

It is an Ericaceous plant, needs to be planted in Ericaceous compost, and fed every 6 weeks with
Ericaceous plant food if you dont live in an Acid
ground area. Do Rhododendrons grow happily in
public gardens in your area ? Did you shovel some soil from your Limestone area garden into the container ?
Easily done.

Put a large saucer underneath the pot to save the
feed being lost. Water gradually, so that the whole
root system is able to absorb the liquid gradually.
Water with MiracleGro (£4 from Wilkinsons) Azalea
Camellia, and Rhododendron soluble plant food.
(Or a similar Ericaceous plant food.)
Make a note to remind yourself to do it again every 6 weeks all summer, on the calendar.
If it does die, get another and follow the above advice.

I'm puzzled at it having flowered so early, here in Devon mine don't flower until August, mine are planted in the ground but with this hot weather of late I have had to leave the hosepipe running on them for about an hour as they need moist roots. Perhaps this is your problem, especially if it is in a pot.

I don't think Celia meant it had flowered this year though. It might be worth checking the trunk at the base - sometimes voles will gnaw the bark of shrubs and if they remove it all the way round this would kill the shrub.
Otherwise all I can think of is weedkiller spray drift as Cammomile says but if no other things were affected it seems unlikely. A puzzle and I'm sure you have everybody's sympathy.

Doesn't look like hydrangea. I think it's a viburnum plicatum cultivar. Look at the distinctive layered branching. I think that possibly it got dry, and collapsed. They hate drying out, and will fail like that inside a day. Water heavily. It won't do any harm, whatever the cause. And even if the flower is lost the shrub can recover.

Gosh Worthy it certainly doesn't look like a hydrangea does it? And if it is a viburnum that would explain why it has flowered already this year, if that's what Celia means.

Thank you for a fascinating range of answers to my query! First, let me assure you that it was, indeed, in flower here in West Cumbria when I photographed it. Then, it was acquired as a lace-cap hydrangea, years ago from the late Ron Sidwell of Paris, a hillside hamlet near Evesham, author of West Midland Gardens, published in 1981. Now I'm beginning to wonder if Ron sold us what we THOUGHT was a hydrangea because now you mention it, it certainly looks more like viburnum plicatum! Our garden is walled, densely planted with a mixture of shrubs, roses and perennials. The late lamented 'hydrangea' flourished happily here for years, in a bed where things like rhodos and camellias also thrive. We can also grow meconopsis in this soil. N.B. that no weedkillers are used in this garden and, being West coast, it's generally damp. As to whether I can convince my 'other half' that what he has been referring to as 'one of Ron Sidwells hydrangeas' for so many years is (was), in fact, masquerading under that name! Thanks again. Celia