Correct way to wire

It should work OK with any of those connection methods.

Which is best depends on any physical obstructions e.g joist or beams that may be in the way of your cable runs and whether you have a fan that is required to 'run on' for a period after after the light has been switched off.

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I am glad you mentioned the run on timer as I didnt take that in to account. That may explain something I couldnt understand. The wiring is in a bathroom (ground floor) and I have taken out the hologens to replace with LED, I came across a cable that is live 3-core + earth that doesnt switch off at the light switch, currently there is no extractor fan but where this cable is located it could have been its intention or maybe had a fan at some stage, as I would need a live feed for a run on timer fan should I use this cable solely for the fan?

Yes i think that would be best, although ideally you should identify where that 3 core is fed from i.e which fuse or circuit breaker controls it so you know if its off a power circuit (>10A) or a lighting circuit (

Its off the ground floor lighting circuit.

That should be OK then.

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Just to confirm was that THREE cores plus earth, i.e. red, yellow and blue or brown, grey and black?

If it is and was originally used for a fan with run-on then I'd expect the three cores to be permanent line, switched line and neutral. Having found that one is permanent line then it would be worth checking to see if one of the others is in fact switched and that the remaining one is neutral in which case you could extend this cable and use it as the feed for the fan.

Also note that if you want to wire the fan for run-on you need a three pole isolator switch and not the two pole one shown in your diagram.

The isolator must be outside zones 0,1 and 2.

Zoning noted.

The wires are Blue, Red, Yellow + earth

No worries I always check and double check when doing anything electrical as it scares the crap out of me since as a 7 year old 50 odd years ago I stuck my fingers in to a lamp socket after taking the bulb out, I have never forgotten that.

One of them is permanent live but does one of the others become live when the light is switched on?

If the cable was once for a fan and had followed what I'd regard as intuitive choice of colours I'd expect red to be permanent live, yellow to be switched and blue to be neutral.

I will check that out tomorrow and report back.

Thanks

Does A actually contain a neutral?

Most light switches have a live and a switched live (with the blue wire used as one of them). i.e. they are not the source of power to the lamps.

See:

John, My intention was take out the 4 halogen lamps and when I find the power feed for those halogens I would then just connect a ceiling rose for some LEDs, I am currently working in the void via the halogen holes in the ceiling plus a couple of other holes I had to make, I then decided I need to get an extractor fan (bathroom)and at this point (on paper) was quite happy with my intended wiring. As was then pointed out to me if I install a run on (timer) extractor then that changes things, something I hadnt taken account of. I need to spend a bit more time checking out the original wiring based on what I have read on here and if necessary more of the ceiling will have to come down (I was only avoiding this due to possible asbestos built late 80s)

In other countries it seems quite acceptable to clip wire to walls and ceilings. Its not as if we generally hide central heating pipes.

In keeping with the UK tradition of hiding wiring, I might put cable in a small box in a corner of a wall and ceiling.

Yes, you will need a switched live for the fan, and I believe a 3-pole isolator as well. I'm not sure if that is mandatory?

Sorry, unswitched (permanent) live as well as switched.

"If the cable was once for a fan and had followed what I'd regard as intuitive choice of colours I'd expect red to be permanent live, yellow to be switched and blue to be neutral. "

OK I untaped the wires and this is what I found, The red and yellow were twisted together. blue seperately taped. Using a non contact mains tester, Blue dead, red always live, and yellow live only when light switched on.

How confident are you of your non-contact mains tester?

It is unusual to see the r-y-b colours for a non 2-way lighting circuit or one that didn't have a time-delay fan installed.

Are those same colours in your light switch?

If the switch only has a single twin & earth cable then the switch drop would have come from junction box or loop-in ceiling rose. A 3 core and earth feed could have been taken from either of these.

The switch has Red & black with earth not connected.

Then there is a junction box lurking somewhere.

It's very fortunate that you have all the wires you need for your lighting and your fan!