I have some very large and established Hydrangers which seem to have become victims of a disease that creates a discolouration on their leaves which subsequently leads to the death of the plant

Possibly Hydrangea leaf spot, but I didn't think that is a killer.

Likely Cercospora hydrangeae, a fungal leaf spot infection, worse in wet weather. It spreads by water splash, so if you've had lots of wet weather, that will encourage it. It also overwinters on fallen leaves, so you should clear those away in autumn. If you water the plants when they're dry, try not to get water on the foliage. One of the problems with very large specimens is that, during wet weather, some of the leaves are crowded and stay wet or damp for a long time, which also increases the problem. Selectively pruning out at the base some stems here and there to increase air flow might help.
Fungicides available in the UK are pretty much ineffectual for this, but you can try giving the plants a feed - nitrogen helps the plant to fight the infection, so use a balanced feed such as Growmore granules. Apply now and again six weeks later.