Looks like leaf spot so firstly I would cut it back by a third to tidy it up and treat it with Provanto 'Fungus Fighter' or similar and then give it a feed with something like Vitax Q4. I would imagine that you might need to treat on a regular basis though.

Thank you, Jimmy. I'll follow your advice.

Have a look at the RHS site.
https://www.rhs.org.uk/plants/garrya/growing-guide

I did, Sbg, but the lesions on here didn't look much like the ones on their site!