Good morning my name is ***** ***** I will assist with your query today.
Please give me a few minutes to look up your specific model
So the controller display just goes blank when the boiler starts?
This sounds like a short on the circuit board, the switch is most likely faulty.
Are you able to induce it by pushing the switch half way over?
Can you also try the same with moving the HW to continuous?
Ok that proves the fault, its nothing to do with the relay or boiler as the fault would occur each time the boiler started (on auto or continuous).
The cheapest option is to replace the timer, the Honeywell model which replaces your unit is the ST9400C, which costs around £70.
You could try and replace the switch but unless you have an electrical engineer background and handy with a soldering iron its beyond most peoples DIY skills.
Since the fault only occurs on continuous, I might be inclined to leave it until it completely fails.
Give me a minute while I find the manual again.
You can set the switch to "Once" - The heating will turn ON at the first ON time set and stay on until the last OFF time set each day.
If you just want to override it, you can press the "change" button to the right of the switch, which will turn the CH on and stay on until either the timer tells it to switch off or you press the button again to manually turn it off.
Its not a big job changing it, just make sure you switch off the power to the boiler (as that is what serves the timer.
I know Screwfix are selling this unit for about £80 if you want to pick it up today.
I wont blame you when you look at their prices!
Appreciate if you can rate the service received today.
A quick google showed you can pick it up for £61.50 from here, including delivery or on ebay here for £58.99