Immersion heater

[AnonymousImport]

The switch for our immersion heater has burned out (35 years of service, not bad!) and has to be replaced. I've got a new 20amp dp switch, but can't wire it up to the load and supply cables because they're doesn't appear to be any slack on either of them; they're both deeply set in a wall and are awkward to get at. The different cables go different ways too, ie the supply comes from below, while the load goes upwards.

 

[Hamiltonandy]

This is why you should always do wiring inside conduits. I use Ega oval plastic conduit which fits into holes in metal box in wall.
There are ways such as chock blocks and short lengths of cable to new switch. However I would prefer to recut channel and put new wiring in conduit. Your wiring is probably getting on a bit so would be worth effort replacing.