Dodgy Hydrangea foliage.

Any ideas as to what is wrong with this Hydrangea? Same watering regime as others that are fine, this one always seemed a bit pale so I sprinkled on some growmore and it's had 2 or 3 liquid feeds over the summer. It was only planted last year and has in fact grown and flowered well, a recent storm has wrecked it's shape now though.

 

It looks like interveinal chlorosis.
The causes and cures are in this RHS article-

Chlorosis / RHS

 

Yes - I agree, it looks very chlorotic.
Is it in a pot? That's always harder for maintaining their health.

 

Yes, in a large pot in a 50/50 mix of topsoil and MPC

 

They don't need as much food as you've been giving it, even in a container. The important thing is the content and structure of the soil mix, and if you add more organic matter over winter into spring, that should help, but some of the composts are poor, and either don't retain moisture well or do the exact opposite. Compost always breaks down further too, so pots need topping up regularly.
I'd use mainly soil but with a bit of fine gravel or grit mixed through it if I had to grow a hydrangea in a pot for any length of time. That gives a good growing medium, and ensures drainage when it's dormant and rain gets in more easily.
How long it'll be happy in the pot will also depend on the variety. There are some newer types that stay smaller.