Well that was interesting!

[Cousin Jack]

Bathroom fan not running. No surprise, the builder fitted crap cheap fans, so off to Screwfix, buy a half-decent one, and a quick change out. Easy job, one hour max. Testing? No need, another fan has failed (again), no testing needed.

Buy fan, fit fan, and voila! It doesn't work, so start checking. Live OK, Neutral OK, no Switched Live.

Off with the isolator, all terminals ok. Live, yes, Neutral, yes, Switched live, no.
Faulty isolator? No, same results on the input side.

Oh deep joy, off to the loft and start looking for cables under the insulation. Not quite impossible to get at, but close. 16amp box with more cables than you can shake a stick at. Fan isolator (obviously), 6 downlighters, switch, and live spur from a ring somewhere. Check box, Live, OK,
neutral OK, switched live, OK. Must be the termination in the box, so unravel all the spaghetti, re-terminate everything carefully, and off down to check.

Zilch, no switched live, and I am reluctantly forced to admit I have a cable that is broken, so, after a swift visit to B&Q to buy 10 metres of 4 core I am pulling, tugging and fishing to get a new cable through an inaccessible hole in a loft space that is hot, and full of scratchy insulation. Re-terminate that whole nest of spaghetti AGAIN, re-terminate at the isolator and voila! It works! A 1 hour simple job took 2 very sweaty days.

I have been DIYing all my electrics for 50 years now, and I have never before had a cable with a broken conductor without serious mechanical damage to the cable sheath. Especially in one that has been working fine for about 5 years and is in an inaccessible place with no possible interference.

Bloody gremlins innit!