How long has it been growing and what type of soil?
It looks like it might benefit from being cut back and rejuvenated.
The best climbing hydrangea I ever saw was in the scrappiest bit of dirt, less than the size of a single paving slab, at the foot of a barn wall.

They prefer an acidic soil and they don't like being over fed but if it's been in the ground for a long time it might be worth giving it a dose of camellia/rhododendron/azalea fertiliser. Use a slow release fertiliser now with another dose in a couple of months if the leaves haven't fully greened up.

Thanks urbanite, agree that it could do with a feed as I've never fed it and it is looking very woody. It must be 30 years or so and could probably do with a little tlc!

mine sometimes throws up an all white shoot, DNA problem so it doesn't produce the green pigment. I leave it for a bit let it flower then prune it out.
I chose it because it dint need acid soil according to the sources I read.
mine has been growing in chalk soil for 20 yrs and every year it gets a sprinkle of grow more or chicken manure pellets. OH prunes it back when it gets to the gutters or cuts out the light from the window it grows across.