Russ wrote:

The only thing I can imagine is that the new thermostats (I've tried
2...the first one was really crap and kept tripping all the time)
simply don't heat the water enough, or the hot water at the top of the
tank is cutting out the heater, so the majority of the tank is still
not being heated?


Any thoughts / advice would be greatly appreciated.

It does sound as if there is a spectrum of temperatures from the base
of the tank
to the top of it.

Whilst not a solution to your problems, how about investing in (say) a
digital
thermometer and measure the temperatures every ten minutes at different
heights
of the hot water tank?
This might give a clue to where the problem lies.

HTH

Mungo