Mira Zest trouble


I have an old Mira Zest 8.5kW shower which has recently stopped working. (see http://www.mirashowers.com/onlinecatalog/pdf/p3617_1.pdf for pdf of manual)
I dont want to buy a new/replacement shower as I will shortly (within the next 6-9 months) be redocorating the bathroom, including a new shower.
The shower is an electric one which has 3 different "power" settings; Cold, Medium and Hot.
If I turn the power on via the pull cord, the light on the pull cord switches on - showing that electric is getting to the pull cord.
Now, if I turn the shower on with the power setting set to Cold, the red light stays on. Note that the first setting, Cold, can be obtained without electrical power (quoted from the manual).
If I now turn the shower up to either Medium or Hot the red light on the Pull cord switches off, and I get no hot water.
Now, if I turn the water supply off and leave the electrical supply on, the light on the pull cord always stays on, no matter what the power setting is set to (i.e. Cold, Medium or Hot).
This leads me to believe that the Microswitches inside the shower are working fine (otherwise I would expect the light on the pull cord to turn off regardless of whether the water supply was on or off), and that the fault is either the thermal switch, or the heater tank. It appears that the fault only occurs when the shower actually attempts to heat the water it is pulling through.
So, my question is (finally!), how can I test if the fault is in the thermal switch, or the heater tank - bearing in mind that I am not advanced in fault finding, but do not mind trying! Obviously, I dont want to turn the electric and water supplies on and then stand in the shower with the case off, trying to fault find with the multimeter... That sounds rather stupid!
Thanks in advance, Durn
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Fit a new pull cord switch!!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I did and it worked! Tested the old one with a multimeter, and the voltage going in was 240v, the voltage coming out was 30v! No wonder the light kept switching off - thanks for the suggestion
Durn

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
No Problem Durn

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.