Wiring was done by a plumber not a sparky.
That is soooo pik ey.
If I had wired up a system like that as an apprentice, I would have been slapped round the head and told to do it properly!
Had one for years now. Wouldn't be without it.
https://www.electrical2go.co.uk/dade-dust-and-debris-eliminator-dade.html?gclid=CjwKCAjw6dmSBhBkEiwA_W-EoCm4Fkqg1LmSEfml2vKbafqb7pTqy_V0RbPjI2EZZK3Ry1Q2WPQjbhoCv5EQAvD_BwE
The screws are adequate to fix the plates, no pilot holes required. The original screws fix the hinges to the plates.
I have done several of these repairs and not had any fail yet.
Just change the tap tails for ½" ones.
https://www.screwfix.com/p/flexible-tap-tails-10mm-x-15mm-x-300mm-2-pack/4565g
https://www.screwfix.com/p/flexible-tap-tails-12mm-x-15mm-x-300mm-2-pack/5891g
I would suggest L1 and L2 go together, that is normal for UK use. In EU they have 16A circuits hence the two lives, one per 16A circuit.
Do you have 2 wires for neutral as well?