When the die back occurred, did the tops of the affected branches hang over, like a shepherds crook?

Yes, they do and I have since learned that the problem may be caused by "fire blight" Do you agree?

No, you don't get 'shepherd's crook' drooping with that - fireblight, which these plants are prone to, gives the appearance that parts, or all, of the plant have been burnt - the leaves go brown and crispy, the wood may go orangy brown, etc. What shepherds crook drooping is indicative of is fungal infection, usually phytophthera and usually at the roots, which is, frankly, just as bad as fireblight in that the plant usually dies. Cut back all affected parts and hope it survives - if it doesn't dig it out next year and don't replant the area with hard wood shrubs for a year or so, use soft stemmed bushes or annuals instead. If that area is particularly wet, or the drainage is poor, try to improve that situation before replanting a new shrub or tree.

Dear Bamboo,

Thank you for your most informative answer. It would appear from this that part of one (mature) has/is suffered/suffering with fireblight. We have had a particularly dry year in Leicestershire, could this have been part of the cause? Also, is it possible for only a part (say up to half) of the plant to be affected whilst the remainder appears to be healthy? As mentioned before, I have cut away the "dead" wood" and disposed of it.

Usually it affects part of the plant and gradually, over a time, the whole thing dies. But if you're sure its fireblight, you ought to remove it instanta - it will spread to the others if you do not. And bear in mind that fireblight infection means you cannot plant anything else from the Rosaceae family of plants - its a big group, including Photinia, Roses, Pyracantha, anything in the Prunus group, including flowering cherries and cherry laurels, plus others. If its phytophthera, then there isn't a blanket ban on any group of plants, just a problem with the soil where the plant has been.