There should be a ¼ Turn shut off valve, on the cold mains supply to the boiler. You will find this in line with other shut off valves , at the bottom of the boiler.
Are you sure you've fully isolated the electrics to the boiler. Re-Check !!!
Possible causes - Faulty Pressure relief valve...
The White Wires are L n N to the Lamp Holder.
For decoration or changing the Lamp Holder, these wires can be disconnected without the need removing the back plate/ceiling holder.
It then can be capped, using a Blanking Cap, so making safe untill future access is needed.
I have a similar hedge and it gets a trim a couple of times in the middle months.
No problem with an electric hedge trimmer, but you don't want to cut right back, like you do a Privet Hedge.
Try to keep to the new green shoots, not the brown branches. Tops can be cut right back as you need...
There are flexable pan connectors out there with 4” - 3½” End Spigot adaptors.
I've used them when replacing the old Sypnonic WC Suites with Modern WCs, Just can't remember who made them.
For those who have not come across old Sypnonic WC Suites, their outlet was 2" or 3" below todays WC Pans.
Was the new olive made from Brass ?
Sometimes a Copper olive, in this situation, maybe better to use as they are not normaly as wide as the brass ones.
This inturn will give the nut more distance to travel up the thread of the fitting body, making a full compression joint.
Before making...
Trying to explain your problem in words may not give the full question, therefore interpretation of the problem may not give you the correct answer.
So to help in a situation like this, a photo or photos may get you a remedy for your problem.