45A, Shower switch, overheating, & crumbles into dust eventually.

Problem discription

1 I have upgraded my shower from 8.5 to 10.5kw.
2 The shower switch now overheats and disintegrates over time. I have replaced the switch 4 times due to this. (I am currently trying a 50A switch but not sure this will make a difference.)
3 The longer the shower you have the hotter it gets and smells like burning plastic after every shower.
4 The switch is a single gang double pole 45A with neon outside the bathroom.
5 I think the cable from the shower to the switch is 6mm and is no longer than 3.5 meters.
6 The cable travels through the walls (so is insulated by the surrounding plaster).
7 The fuse at the mains is a seperate 45A switch but it is not a modern RCD type.
8 When wiring the unit up I have made sure on this last occasion that the wires were untwisted at the end and screwed down in the unit very firmly. I have cleaned the end of the wires with grit paper as best I could.


questions?
When wiring this switch there are two earth wires. On the diagram it shows the wires going to two separate connection points. But with my wiring the two earth wires have been twisted together and attached to upper earth terminal.
Could this be the cause of the overheating?

question 1
A Is it ok to have the earth wires together like this or could this be the cause of the overheating?
B If part A is ok does it matter which of the two earth terminals the earth wires are attached to?

question 2
will I need to upgrade the cable to 10mm and would this be the cause of the over heating?

Thankyou for any replies all help and advice is much appreciated.