That looks viral. There are multiple viruses that can affect lilies.......and none of them are treatable. The recommendation is to destroy (not compost) all affected plants.

Ok thanks for your reply I had got that in the back of my mind actually but was hoping it wasn’t. Would that follow as for the past 2 years it’s been much larger and flowered beautifully? Can they ‘catch’ a virus like we catch the flu or is it there from the outset??

They can "catch it".....but not exactly in the way you think :-) Typically diseases like that are vectored by an insect that feeds on an infected plant and then transmits it to a previously uninfected plant. It is very commonly transmitted from Lilium lancifolium, which is a common host although usually unaffected by the virus itself