Many thanks for the useful advice.
The temperature difference was 22c (77c vs. 55c). I've cleaned out the heat exchanger and now I have hot water (as well as central heating).
Once again many thanks.
I've read a number of threads and, whilst there are number of similar faults listed, I can't find anything that matches my symptoms exactly.
My boiler has recently developed a fault where the domestic hot water alternates between hot and Luke warm. The central heating works fine.
When...