I would have expected a fan fault (f7/8) to cause an issue with both HW and CH, you may find the boiler is not working efficiently and therefore you not noticing it with the CH, but the HW is more noticeable if it doesnt get up to temp.
I would certainly look to get a gas safe engineer out to check the boiler, clear the fan fault then see where you are, since the only other fault I can foresee is a failed solenoid or valve, but again that would still need a gas safe engineer to strip down the boiler.
Should you wish me tor recommend someone in your area please provide me with your postcode.
Due to the time of year your looking at around £100 an hour to get an engineer to attend.
I would recommend using Plumbforce on 01246 384470, they normally have same day call out available.
If you can wait until next week you can try Lowe' Services on 01773 300681, they charge around £65 an hour.
Please let me know if you need any further assistance.
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