A sudden drop in flow rate to a specific part of the premises plumbing is usually associated with something being altered,such as an isolation valve being partially closed,pipe being crushed,or a physical obstruction restricting pipework.
Could any work have been carried out on your plumbing in...
Do you mean one bulb works in all positions ?
If so connect the permanent live to the switched live at the switch position and see if all lights work. If they do the dimmer is the issue.
You could check tomorrow how well the olive sits on the pipe by undoing the compression fitting,and try to knock it off the pipe .
Rapping a spanner against the nut onto the olive. If it slides off the pipe easily it's a weak joint.
I assume the pipe is under mains pressure ? If it is, close your stopcock overnight if you are concerned.
It sounds like you didn't tighten up enough when it blew apart.
Old imperial copper is thicker walled and often harder too. You therefore need to tighten up the compression fitting quite a...
Looks like at the very least need to strip out the radiators bleed valve ,and valve tails and re dress the radiator. If finances allow ,consider a new radiator and valves.
The condition of the pipe/ pipe ends must be in good condition ,no scratches on the pipe circumference,and pipe end should be smooth and free of burrs. Lubricant would ease assembly.
As well as the points noted above ,there also appears to be staining on the end panel of the radiator. The bleed valve ( or blanking cap ) at the top of the rad may be leaking.