Thanks for reply Brian.

"Brian Sharrock" wrote:
"Steve Gouldstone" wrote in message
.. .
My hot water tank has wiring for an immersion heater, but one isn't
fitted. The tank, I would guess is about 20 years old.

I don't understand what you've written: - "wiring for ... "
but "is not fitted ... ".

A hot water cylinder - 20 years old may be 'naked'
copper with an external insulation blanket or foamed
insulation clad - should have a large boss on the top
for accepting an immersion heater. The immersion heater
assembly should fit directly into, and replace, the boss.
From what you've said - you should see a big nut
hexagon on the top of your cylinder.

Yes I see a big hexagon nut near the top at about 11:30 o'clock which
fits this description of a boss.

Immersion heaters usually, when fitted have a cylindrical
'can' -all those I've encountered have been black -
normally secured by a knurled nut off-centre within the
'can'. 'Wiring' normally makes contacts with terminals
under the can. It's fitted on an installation -specific
basis to the feed/source of the 'immersion heater' supply.

Where does your 'wiring for ..' start and finish?
what is on the hot-water cylinder top?

Sorry, I could have been clearer. There is a mains wire going into the
airing cupboard, but it terminates in a thermostat attached to the
outside of the tank. There is no output cable from the thermostat.
Presumably this is meant to continue on to an immersion heater?

The nut on the boss is slightly bigger than my biggest spanner, so
I've not tried to remove it, also I don't want to introduce a chance
of a leak until I've got a fair idea I can just put an immersion
heater in there.

Steve