It looks like lacebug damage (not to be confused with lacewings!)

Capsid bug or thrips I'd say - we don't have a lacebug that affects clematis in the UK, the only one we have is Pieris lacebug. Flea beetle is a possibility, but whatever it is isn't living on the plant now. Try spraying with an insecticide that professes to deal with these next year if it happens again..

Thanks. I checked out all the likely suspects suggested & the one that matches the damage on my plant is thrips, so thank you, Bamboo, I hadn't thought of that one.
Bayer Provado Ultimate Bug Killer or Bug Clear Ultra seem to be the best remedy.

Both of those are neonicotinoid based products, Greenfinger, so use them by all means, but try to spray when it isn't windy, and at dusk when the pollinating insects (bees, hoverflies) have stopped working, and keep the spray off any open flowers nearby.

I don't know which type of Clematis 'Westerplatte' is, but if it is still trying to bloom, I would pinch off any flowers or buds for a month after spraying to protect the local bee populations.

Bamboo, I prefer not to use chemicals but if I need to I am always mindful of the beneficial insect life, appreciate that you care too.

Tug, Westerplatte is a group 2 large flower type & has finished flowering. It's looking sad since the bug damage so is unlikely to bear a later flush. The Duchess is a group 3 so will be hard pruned later but has not flowered at all, it doesn't look happy either.