CH ok - DHW ok with CH on, if no CH on then DHW only lukewarm and needs taps wide open to make boiler fire, as soon as demand is slightly decreased then boiler stops.
diverter valve faulty. will need new one or re-condition old one. not really a d.i.y. job. you should call an engineer and check if they will try to repair it. some will some won't.good luck
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