Triton Stabiliser Valve/solenoid Assembly P12120801

Unfortunately there is nothing that is a direct swap.

However....... Triton has recently brought out the T80Z this shower has been designed round being a retrofit unit  (Tritons word not mine!)

The pipe work can be brought in from almost any position and there is a reasonable amount of space for cable entry from most positions.

Here is the link.

http://www.showerdoc.com/new-showers/triton/electric-showers/ZTRITO-SP8008ZUFIT-Triton-T80Z---8-5kW-(White-Chrome)

See Video at the foot of the page

If when you switch off the shower at the main isolator and the shower keeps running then you have a faulty solenoid.

When you switch off the shower at the main isolator  and the water stops  running, then you have a faulty on/off switch or a faulty PCB or a micro switch that is sticking on.

Carry out the above check, however from what you say I think you have a faulty valve assembly.

The things to check first are.

1 That the main valve coming into your house is fully open.

2 Then check that the valve on the line to the shower is fully open.

If these valves are not fully open then there may not be enough water
getting to your shower to cool it properly the overheat safety device will turn
the power off until it cools Then it all starts again