I would check the soil pH to see if it is too high and amend the soil if the test confirms it. I would also check to make sure that it is not getting too much moisture and a soil test to see if the levels of nitrogen are too low or high. Plant nurseries should sell soil testing kits to do check the pH and NPK. The shrub seems to be a Hydrangea arborecens like Annabelle.

Health looks normal for an Annabelle. The more shade the brighter the leaves. They aren't native to the UK so might not like the maritime climates per se.

Thank you both for your answers. You may be right Springwood_Gardens about it being normal, but it has looked better the last couple of years. We will buy some soil tests to check your theory Luis_Pr.
Thanks again, Hemley