I honestly don't think it will be the power supply in your house. You are using a surge protector, but if you are in a UK town there is unlikely to be a problem except during thunderstorms. It is more likely either bad luck to have two similar faults, or possibly vibration or handling (I mean, something might have come loose in your new PC as it has recently been in transit - unless you are standing it on top of the washing machine or something ).
It is possible to have a new display not work because the Refresh Rate is wrongly set, but in this case (in my slight experience) you see the power-up text when your turn on the PC, before Windows starts. It is also possible for the plug or cable to the display to come loose, but your helper would have spotted that, I'm sure.
If you have opened the case and fitted parts yourself it is worth using an antistatic wrist strap, especially if you have nylon carpets or rubber soled shoes. It is also possible to disturb plug-in parts when fitting new cards.
I hope the supplier either replaced or repaired your new machine?
It is possible to have a new display not work because the Refresh Rate is wrongly set, but in this case (in my slight experience) you see the power-up text when your turn on the PC, before Windows starts. It is also possible for the plug or cable to the display to come loose, but your helper would have spotted that, I'm sure.
If you have opened the case and fitted parts yourself it is worth using an antistatic wrist strap, especially if you have nylon carpets or rubber soled shoes. It is also possible to disturb plug-in parts when fitting new cards.
I hope the supplier either replaced or repaired your new machine?