Not a professional myself, but with much experience and knowledge
gleaned from troubleshooting my own CH & HW system, the following
springs to mind (please correct me if I am way off the mark)...

1. Has the property owner noticed anything abnormal with his/her CH &
HW prior to spotting the discharge from the overflow or does the rest
of the system work as it should?

2. Have they had any additions/changes made to the system recently? For
instance: my dad recently fitted both cold AND HOT outside taps to his
property so that he could wash his dogs after a muddy walk. He attached
a cheap 'mixer' hose with shower head to the taps and as a result, the
pressure of the cold feed sent water back up the HW system and led to
discharge from the storage tank overflow.

3. Are any of the radiators blocked, causing the CH water to circulate
much quicker, without raising the room temperatures and therefore
activating any roomstats to switch the system off?

4. Are any MCV's malfunctioning?

5. Is the cylinder stat faulty?

6. Is the immersion heater faulty?

Overall, are you able to confirm that the HW stops being heated when
told to do so, by other components within the system?

Let me know if any of the above is a potential culprit.

HTH,

deano.