Could you just add a large immersion element to the tank to heat the water
one that you could turn on and off as you need it
Dean
You could. It would have to be made of inca-alloy or stainless steel though.
www.andwat.co.uk sells them. They drink the electric as he sells the 3Kw version for around £300 inc VAT & Postage I think.
If you leave it plugged in too long and the water in the tank gets too hot you'll distort it out of shape and eventually it will leak (with constant overheating).
Also beware you don't draw too many amps from your 13amp extension cable making sure it's fully unwound, otherwise you'll overload it because most are only rated at 3120 watts (unwound)and the heater draws 3000watts.
Also beware if you plug it into a socket that has been made a spur (extension socket in the wall from an original) otherwise you could be drawing too many amps
elsewhere without realising it.
It will heat the tank sufficiently but it's an expensive way of heating the water.