that looks like lily beetle damage.
under the leaf there will be slimy blobs of lily beetle larva covered in their own poo.

The adult beetle is a glorious red colour. When you try to pick them off they fall off onto the soil on their back. so you cant see it. Place a hand under the leaf the adult is on then move to get it with the other hand. squish it.
as for the larvae, i wear gloves and wipe them off, rinse them in a jam jar and pop them on the bird table.

Thanks. Any ideas on how I can prevent this happening next year ?

other than not growing them all you can do is to keep an eye for the adult beetles. I regularly [daily] check them. There are insecticidal sprays but I wont use them as I am encouraging wildlife into the garden.

As SBG says, daily checks are the only real prevention, easy if only a few lilies in a convenient spot

Being systemic The sprays are useful against the larvae, I am using them for first time this year but will have to stop soon before the flowers open, don't want the bees having a slurp of it

There is also a vicious pleasure in squirting a red beetle going about it's business but out of reach

Thank you both. In other Lily related matters ....

Will it harm them to cut them right back now ? Also I want to relocate one set of plants. When would be a safe time to dig up the bulbs to move them ? Thanks in advance (once more) Peter

if you cut them back now the bulb and flower for next year will not form very well. I usually lift them as the foliage is going yellow and the plant dying back, usually autumn.

picking off some damaged leaves wont hurt it though as long as it is not more than half.