First thing to try would be to let the entire central heating system cool down, then set central heating to off, (or choose a time when central heating is not programmed to come on), then turn on a hot tap to fire up boiler. Leave tap running for a few minutes, then feel the central heating output (CH Flow) pipe coming out of boiler. If this pipe is heating up, your problem will be a faultly diverter valve within the boiler. Try this and let us know if CH flow pipe reamians cool and we will need to look at other possible causes. I suspect that your boiler could be 20 years old and parts may be difficult to obtain.
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