Either drought (though if the plant is mature, unlikely) or the cold weather has nipped the new leaves and they've died off. Shouldn't be a problem from now on - it's roasting here in London!

Up here too!

Must have been the cold nights as mine did the same, beginning of May was worse then April had to have Central Heating on in the evenings. We cant win can we with the weather, now this sudden heatwave, just hope it's not going to change again suddenly, that really would be catastrofic. It happened before one year when the weather in part of June was dreadful and very cold

many thanks. I hope thats all it is