The ideal solution is obviously to run new cables. However, the cables could be extended (e.g. by crimping, which an electrician could easily do). Ordinary junction boxes which are inaccessible (e.g. 'hidden in the wall') are not permitted. However, if it is a hollow wall, you could theoretically use a 'maintenance free' junction box (I personally would not be happy with such a box 'plastered in' to a solid wall) - but most would probably regard that as a pretty poor practice.
Depends on the type of cable.
True - that's why I wrote "e.g. crimping" (which, as you know, is not my favourite technique!).
If I recall correctly, the original specification for the Ashley/Hager MF type junction boxes, prohibited them from being buried within a solid wall/floor. So not just bad practise (Disclaimer:but other MF junction box manufactures may take a different approach or Ashley/Hager may have changed theirs?)
If you read carefully what I wrote (and you quoted) (i.e. temporarily ignore the bit in brackets), it was the burying of an MF JB in a hollow wall that I said that most people would have regarded as 'bad practice'. The bit in brackets indicates that I would not even think of burying one in a solid wall, and I can quite believe that the manufacturers did/do say that this should not be done.
Fair enough. As a statement of the obvious .... if you are going to crimp or solder and then bury those joints in a wall, you need to make sure that all the conductors are adequately insulated (from each other and from the surroundings).
It's probably advisable to also put a further heat-shrink over the whole lot - thereby emulating the 'double insulation') (insulation plus sheath) that exists in intact cable.
Do you also have, the correct to check once you have crimped the joints that the joint/circuit is satisfactory. I would also recommend the crimped joint is has heat shrink applied.
I often point out that the sheath is not proven as an insulator and therefore insulated and sheathed cables should not be confused with double insulated.
Yes, you do often point that out and, whilst it is strictly true, I have little doubt that the sheath does, in practice, provide effective insulation (at 230V).
You are undoubtedly correct in implying that a single layer of heat-shrink (some people use two, or even more) does not provide as much 'mechanical protection' as does the sheath of T+E cable, but I'm not so sure about it 'not being adequate' - since it's not really clear what, if any, 'mechanical protection' is required by a joint buried in a wall.
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