INSTALLING A SHOWER

[riveralt]

If I installed a 10.8kW electric shower with 6mm2 cable instead of the correct size 10mm2 & the correct loading MCB, is it a dead cert the cable will overheat & burn the house down?

Nothing is a dead cert but I would advise you not to gamble.

How easy is it to chase the existing cable (6mm2 with the correct size (16mm2).

Why have you suddenly gone up to 16mm2 unless you are going through loads of insulation etc it is not necessary. Chasing will depend on the route your current cable takes and the route you choose to take.

The property is a newish build 10 years old Bellway; would this be a bu**er to do, or should I leave the whole job to an expert? Thanx.

To me you have three choices:
1. You could get and fit a shower that meets the cable's current carrying capacity say 8.5kw (where there are no issues with insulation)
2. Or you could install the new 10.8kw shower yourself - after of course informing Building Control and paying their fee.
3. Or you could ask an electrician to do it for you.
If you feel confident then 1 & 2 apply if not then 3 applies.