Please could someone explain what is wrong with this Spirea and how to remedy it
This looks like Fire Blight, which is common in Spirea.
When it rains in spring, spores get bounced from stems and branches onto flowers and new growth, infecting the tissue and starting the process over again. Fire blight is cyclical, with alternate plant hosts, so it may not even be a problem most years.
In your situation, I’d recommend pruning out the affected areas, making sure to clean up any leaf or branch litter around the plant. Late winter next year, check the bark for any oozing cankers, and prune out those areas, making sure to toss all of that infected plant material in the garbage.
With regular checking, you should be able to get the issue under control and make it manageable, without having to do anything more than cultural maintenance.