Why is my neutral wire at the consumer unit showing a temperature of 70°C?

Finding that a neutral wire in your consumer unit is running at 70°C is a warning you should take seriously. That temperature is the maximum that standard PVC-insulated wiring is designed to handle, and reaching it in a connection point rather than along a cable run usually means something has gone wrong at the terminal itself. What is actually happening is that electricity is meeting resistance somewhere it should not. When a connection is loose, corroded, or incorrectly terminated, it creates extra resistance at that point. Resistance turns electrical energy into heat, and that heat builds up in the terminal and surrounding insulation. Left unchecked, this can melt cable insulation, ignite the plastic housing of the consumer unit, or cause arcing that starts a fire inside your walls. Things that can cause this to happen include a neutral bar terminal that was not tightened properly during installation, a conductor that has corroded over time, strands of multi-strand cable that were not all captured in the terminal, or a high-resistance joint created by an incorrect connection method. What you can safely do yourself is limited to a few straightforward steps. You can switch off the affected circuit at the breaker. You can feel carefully around the outside of the consumer unit for warmth, but do not open the consumer unit cover yourself. You can also look to see whether any breakers or the neutral bar area look discoloured or smell of burning plastic. You should not attempt to re-tighten terminals yourself. The consumer unit contains live parts even when individual breakers are off, because the main incoming supply and the neutral bar remain energised. Working inside without the correct training and tools is dangerous. Call a qualified electrician, ideally one registered with NICEIC, NAPIT, or ELECSA. Tell them the temperature reading you have seen and describe any discolouration or burning smell. This situation warrants a same-day response. If you can smell burning or see any signs of scorching, treat it as an emergency, switch off at the main switch, and if in doubt call 999 or your DNO emergency line.