I suggest you get an electrician. I would think something has gone amiss with the original wiring hence that yellow wire or the switch needs replacing.
This is pure speculation and without testing I can only assume. Mark that cable, connect the reds together but leave separate. The black from this marked cable is your switched live to the light with the other black neutral to the light. If this doesn't work then;
a) this is wrong.
b) the...
Yes each cable at a time, black and red from one then the other see if either light it. If not the yellow may be in play in either case it may be an idea to get an electrician involved as from a distance it is impossible to be confident with that spare yellow.
Just connecting the light to one set of red/black then the other should confirm the feed, whichever one lights it. No need to touch the switch which may be duff now if it has had a short through it.
It blows because you are creating a dead short between live and neutral.
Confirm whether the three reds in the other switch are not connected to the light, just connected together.
So the three reds were connected together but not to the light?
Does the double switch do each light so, one switch one light the second switch the other?
I think you need to elaborate. PEFC theoretically should decrease the further you calculate from the origin all things being equal. What do you mean by the same thing?.
Appendix 15 states cookers, ovens and hobs with a power rating above 2kw should be connected to their own circuits, this is informative and not a Regulation.
What make is it never seen a pre wired extract fan assuming this is a standard bathroom/kitchen fan.
Connector blocks wrapped in tape, no.
You need an enclosure something like this is cheap and simple...