Does look like damage from some sort of leaf miner. The brown tips to the two leaves on the left are probably caused by lack of water at some point, but the whiter damage isn't that.
Control is removal of affected leaves - serious infestation may require pesticide treatment as a last resort. Most of the old recommended ones for this have been withdrawn, so you should look for something that claims to treat leaf miner infestation.
Recommend you cut back after flowering (if it does flower this year) and feed in April and again 6 weeks later with something like Vitax Q4 to encourage plant growth.
Only other thing I'd ask is whether this plant is in a sunny or shady spot?

Thank you very much for your prompt reply

It is in a fairly open, sunny spot.

My Crinodendron sometimes has small dead branchlets. I prune them out. Try Provado Ultimate Bug Killer spray to try and get rid of any bugs in Spring.

This shrub prefers a part shade situation and dampish soil, preferably in the lee of a fence or wall for shelter, so you might be better choosing a different spot for it and transplanting it now.

I've seen these growing in the wild and their preferred position is moist soil in part shade on the edge of woodland

Thank you to all who offered advice. I shall follow it all.

Crinodendron also prefers an acid-based well-drained gritty humus rich soil. Also needs fleece protection from hard frosts and general bad winters. Well worth a few minutes of extra effort in the winter to keep this one going.

The fleece protection depends what part of the country you're in, and how sheltered a spot the shrub occupies - one of my clients has a large one in West London, and it came through 2 bad winters (last year and the year before, not counting this one) completely unscathed, despite sub zero temperatures and no extra protection, Avkg47. And so long as the soil is lime free, in other words, neutral, it will thrive, so not exactly an acid lover, more a lime hater, which isn't quite the same thing.