hi hywel,
is there a powdery mildew on the leaf [the last photo hints at it unless it is just the angle of the light. lack of water might be a cause or the higher temps we have been enjoying. if you follow the flower stem back has it been nibbled by anything?

as you might I have gathered I don't actually know! :o)

Mildew would do that and that is more prevalent when the roots of the plant are dry.

Thank you both. From your comments I think it must be dryness at the roots.
Although we've had rain, it hasn't even wet the ground :(