Good morning, sorry to hear your having issues with your boiler.
When the engineer came out on the 18th, did they advise what they did to rectify the fault?
To understand the cause, please go to the boiler and hold down the minus button for 10 seconds.
A fault code should appear, please right this down and then press the mode button to go back to exit from the code.
I assume you have on demand hot water, and do not have a hot water tank?
Apologies for the delay,
Its unlikely to be a leak, but in the off chance it is, did the engineer not top it up to 1.5 or 2 bar, as 1 bar is the limit at which point the boiler will operate?
It may be worth topping the boiler up to to 1.5 bar using the filling loop under the boiler, then monitoring the pressure gauge each day to see if it drops.
On the control panel when you run the hot water what lights come on, and when you notice a fault (no hot water), what lights are illuminated and which are flashing?
To troubleshoot, we need to determine exactly which lights are illuminated when the hot water stops working.
Secondly, when a fault occurs with the taps off, you need to reset the boiler so you have green lights, turn the hot tap on then check straight away if the boiler lights (even just for 10 seconds), and if you can hear clicking.
This will show if its an ignition fault, or something else causing the problem.
Due to the issue being intermittent, there isnt a straight forward solution I am afraid, but this most likely will require a gas engineer out to repair, but if we can narrow down the fault it will hopefully ensure they get the part replaced first time.
Ok thats the the ignition light, this is a common fault.
Its most likely to be a sensor fault, what we need to do now is wait for the fault to reoccur, reset the boiler turn on the hot tap and see if the pilot light comes on (see the video here).
If the pilot light comes on and then goes out within a few seconds, this will most likely be a fault within the condenser (it may think the condenser is blocked), or the flame sensor (where its not detecting the flame), resulting in the boiler going into an error state.
From what we have proven thus far, I would look to get an engineer back out and follow the chart 4 within section 13.4 on the engineer manual, this will get them to test each component in turn and test the required sensors.