I has this problem with my water tank in my flat.
It had 2 elements in it. One for ECO7, heating the water overnight. Second for the boost.
Rest of the flats in the block had been having problems and nightmares with different plumbs relacing the wrong bits, ordering the wrong elements, sheering the threads, cracking the tanks. So we did it ourselves.
This all depends on the type of tank you have of course and if you thermostats are intergral with the elements, or seperate.
If they are seperate:
To check the thermostat get a mutimeter, set it to 240V on AC (normally top leftish if it has a dial) and put it across the positive and negative terminals on the thermostat while everything is on. If you get nothing, then your thermostat has gone. Find out the make and model and replace. Turning all electricity to the unit off first and carfully swapping the connections over one by one so you don't make any mistakes (or draw a diagram!).
If they intergral (CORAL was my tank make):
Find out the make of tank. Ring the manufacturer, and get the manufacturers elements sent to you (replace both as if one has gone then the other is not far behind). turn off electricity to the tank. Drain out the tank using a hosepipe until the water level is below the lowest element. Make a note of the connections on the ends so when you fit the new one you get them right, and disconnect them. Your probably going to need some sort of element unscrewing tool at this point so make a note of the shape and type and buy one from a plumbers merchant (some will say use some big spanners etc etc, but believe me some boilers need a unique tool, so make sure!). Next SLOWLY unscrew the element, this ensures you don't burr or ruin the copper threads. Remove the element. NOW THIS IS THE TRICKY BIT. SLOWLY feel in the new element to the threads on the tank. If you mess this bit up you'll cross thread everything and the tank and your new element could be ruined! (As happend to TWO neighbours with their plumbers!). Then using your element tool screw in the element gently, keeping it horizoltal and in the threads, not letting it droop drown and therefore resulting it crossed threads and disaster. Do not over tighten as you can cause damage again. Reconnect the wiring to your element as per your wiring diagram you made earlier.
You should have two switches on the wall near your tank, one for the boost, one for the ECO7. They should have a light on the switch to tell you that this circuit is operating (if not then it may be a good idea to replace your current switches with this type). If the light comes on then your good to go! Refil the tank and let the hot water flow! (in the morning obviously
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If it doesn't come on then recheck your connections using your multimeter. Again having it on 240V and putting across the positive and negative connections on the parts you changed to see where the break in the circuit is.
Hope this helps. And my appologies if I've in any way told you how to suck eggs as you did say you were a plumber. However I would no want you to go through the pain of getting it wrong.
P.S. No I'm not a plumber or electrician. Just someone who wasn't willing to pay ££££ to people who didn't solve the problem, or changed the wrong element, or crossthreaded the elements, or cracked the tanks. One caused a flood over 3 floors of flats because he put in a cheap pattern element without an intergral thermostat and after 2 weeks of overheating it cracked the tank!!
Good luck with it.
