My memory may be playing tricks on me, but weren't there extended droughts in '16 and '17, too?

the one I our neighbours garden does the same thing and has done for about 5 years. the dieback does seem worse this year so t could be water or temperature related. don't know what to suggest though except extra water if we get another dry summer. though here in east yorkshire 2017 was quite wet after a dry spring.

Hi I agree, although willows like a reliably moist soil, most also like well drained soil, particularly S erythroflexuosa, so I would think it's a water shortage problem.
I also agree with Sbg about the long hot summer that everyone talks about, in north east Cheshire the long hot summer ended on 1st of august, and didn't start until the last week of June, so 6 weeks maximum, after that it wasn't warm enough to sit out in the evening, and quite a lot of rain, [4 days before the water company, in 1 of the wettest parts of the country, were the only 1 due to introduce a hose pipe ban] Derek.