On 17/01/2016 18:40, Ian Field wrote:
Thinking ahead just in case it blows...............

The issue is length - I could measure the outside dimensions of the
tank, but without taking the element out there's no guarantee there's no
obstructions inside - and I could just measure the element if I took it
out.

The easy way out is to buy one of the shorter folded elements, but I'm
wondering if the effect on convection would be good or bad.

The tank is square, not cylindrical.

Thanks for any help.

The other issue is undoing without deforming the ever thinner copper of
the tank. Heat up the thread section with a blow lamp before attempting
to undo . Moderate torque plus tapping spanner with hammer , before
upping the torque.