Bulgin power plug is no more used these days. Let’s switch to a standard IEC power plug, adding by the way a fuse for a safer installation.
First of all unplug everything going to speaker and wait for few hours. Now you can remove the back grill and unsolder then remove power supply.
Tip: use some cardboard to protect dust covers during unsoldering !
Tip: take photos to be able to put EHT back in place once done !
Remove legacy Bulgin power plug and take your Dremel or equivalent.
And now, back in place. Note the LED replacing the neon bulb.
Another example, with back grill this time:
