If the flower buds dried up or fell off, insufficient water is the likeliest cause - plants will drop or abort their flowers if there's not enough water around.

Here in the desert, Oleanders have the reputation of great drought tolerance, but that is mostly because of their deep and aggressive root system. In containers, we have to water them twice a day, and although they won't need it that often in the UK, it would be easy to miss a critical few soakings in "hot" weather.

I like the way you put 'hot' in inverted commas, Tug - we do get truly hot at times (a peak of 44.6 C on my balcony only 5 or 6 weeks ago) but I suppose, compared to your temperatures, that's nothing....

No, 44.6º C is pretty respectable, even for us, and it must be pretty steamy, too! My condolences on climate change's lovely "gift"!

It doesn't last though, usually no more than a week at most, and yes, high humidity which makes it pretty uncomfortable without aircon. The worst is the temperature doesn't even drop much overnight when its a heatwave...but then I'm on the edges of a very big city,not so bad out in the sticks.