Yup, this was previous owners doing DIY on a major scale... and it not being picked up on any survey
e.g.
How to wire up an outside light.Take one length of three core mains cable, strip to individual wires.
Locate a Wire under floorboards. Strip outer. Cut ONE inner and rejoin with chocolateblock. Use this to T off one of above wires, feeding switch.
Locate another Wire under floorboards. Strip outer. Cut ONE inner (who cares if it's black and you've got a red cable in your hand) and rejoin with chocolateblock. Use this to T off one of above wires, feeding light.
(I think the other feed to the switch went via the light switch in the lounge.)
How to wire up a cooker hoodPlaster wall.
Place stripped live cable alongside live cable from hood, hold in place with tape.
Place stripped neutral cable alongside neutral cable from hood, hold in place with tape.
Place stripped earth cable alongside earth cable from hood, hold in place with tape.
Press this joint into wet plaster.
Skim.
When I changed the light switch in the lounge the Dining Room light stopped working.
The two way lighting circuit ran off a single two core cable. Before you wonder how impossible that is, it had additional feeds from the outside light (see above).